So an amazing thing happens and we locate an Earth-like planet and we also achieve the technology that enables us to travel there. Upon arrival we discover that dinosaurs are roaming on the entire planet. Would this make it impossible for us to live on this planet? Would the presence of these dinosaurs make it impossible for us to establish an Earth-like colony?
Dinosaurs are not bulletproof. They would probably be wiped out pretty quickly, at least the dangerous ones. We've killed all our original predators on Earth, there's no reason to think we wouldn't do the same thing there.
I doubt anyone is going to let their billions-of-dollars mission be endangered by some local wildlife. Especially if it's one-way, which it most likely is.
Dinosaurs? No problem, the prior probe missions and the 'prepared for anything' first wave of colonists, would send a 'bestiary' to the following waves of colonists and to Earth.
We already have the technology to effectively deal with dinosaurs. Sonic weapons, high voltage tasers, sedative darts, outright poison, ballistic weapons, traps, etc. Of far greater concern to settlers would be the biochemicals in the atmosphere and the microbes present (diseases, etc.)
I'd imagine local wildlife (unless deemed intelligent; I defer to Henry Taylor's answer in that case) will be considered a resource boon and not something that would inhibit us. Unlike movies with giant lizards, dinosaurs aren't immune to bullets and can be dropped … if there is one thing humans are good at, it's killing off wildlife.
I'd suspect we'd view them as resources:
Food. As Serban Tanasa points out, dinos taste like chicken! Perhaps not … but if it's biologically possible to use them as food, we'll eat them. Might even open up exotic food trades as people back on Earth want to find out if they truly do taste like chicken.
Research. This would be one heck of a convergent evolution to find dinosaurs like ours on another planet … I'm sure there is much to learn as to how they got there and what that tells us about our own evolution.
Tourism. Be it zoo or safari, people will travel a long ways to see (and shoot) the exotic.
Labor. This 300 foot tree is in our way! No worries, just hook it up to Ol' Betsy (my Brontosaurus) and it'll be gone! We use animals here on Earth for heavy duty labor and transport, there is no reason why we wouldn't use gigantic ones as such.
Pets! Domestication is feasible. Not quite sure what the dino equivalent of Dog is though. I'm sure I'd have little problems selling off a purse-sized raptor that can dance the cha-cha on command and curls up on a pillow beside me at night.
Given our current (and probably prevailing) move towards political correctness and eco-friendliness, it is likely that discovery of dinosaurs on a colony-targeted planet would generate in several light-years of red tape, the spawning of dozens of committees to endlessly discuss the saurian rights and the election of the Geico gecko as pro-tem ambassador for the saurian people.
Colony ships would be required to park themselves in extremely high orbit to avoid detection by the natives. Later, after much debate the ships would be called home until the saurians evolved to a level where first contact would not irreparably damage their indigenous culture.
Far from easing the growing population issues on our small planet, the discovery of dinosaurs would actually add to those problems as samples of the enormous beasts are kidnapped from their home world and brought to Earth as caged exhibits in all the major zoos.
So, yes, the discovery of Dinosaurs would render a planet unfit for colonization, but not for any technical and practical reason.
Peter U, I've got to agree with the previous answers: in a high-tech, well-supplied human-colonists-vs-dinosaurs smackdown, it's pretty obviously a one-sided fight.
However, you have raised a good dramatic question, and I'm not willing to let it simply die out. :-) There are ways to make a worthwhile story of conflict and survival out of this.
Here's one example. (I didn't make this up, really: it's drawn from illustrious predecessors such as Tom Godwin's The Survivors and Marion Zimmer Bradley's account of Darkover's initial population by human starfarers. It also has roots in the Lost World genre of Victorian romance and 20th-century pulps, in which a lost colony of Greeks or Atlanteans or something has persisted in remote parts of Africa or Central Asia...)
Rather than having the "colonization" be an orderly, well-funded invasion-style immigration - the kind of thing dinosaurs could not withstand - you could perhaps have some fun with a shipwreck of a starship. This ship could have been one that was intended as a colony ship of sorts, but not fully equipped to regenerate all of the starfaring technology. This would give you a few plot points in which your human-vs-dinosaur conflict could play out:
Without the ability to sustain high-tech weaponry, medicine, energy sources, and defenses, the humans would be in no position to dominate the world of dinosaurs. Defensive survival would be a difficult proposition.
The humans would be confronted with the necessity of salvaging as much useful material and knowledge as possible from the ship. These artifacts would need to be conserved effectively, and it's pretty certain that the colonists would be slow to figure that out.
Simultaneously, the humans would need to reinvent forms of society appropriate to their reduced circumstance. This would include rediscovering the survival arts such as agriculture, hunting, physical combat with manual weapons, woodworking, artisanship, etc. etc.
These points could breathe a lot of life back into your original concept, I think. You could have a multigenerational saga on your hands. Or, you could set your story point some centuries after the Landing. What kind of world would it be?
Note, too, that when I say "shipwreck" I don't necessarily mean mean a crashed vessel on the world's surface. Shipwreck, in this case, simply means that the ship had to go to an unplanned destination, can no longer depart, and presumably has no reasonable way of signalling for help. Examples:
The ship diverted to a star of the correct spectral type because the engines were breaking down; those engines proved to be unrepairable.
The ship was damaged (possibly by meteors or something) while in orbit, and is no longer livable. The air is gone; or the power systems have died; or there's been a bad radiation leak; or something... The humans had to take shuttlecraft down to the surface; the ability of the shuttles to make further trips to the orbiting hulk is limited.
The ship was overwhelmed by a fight between mutineers and crew; the "colonists" were the losing side. They bailed out to the planet below because they had no other choice. The winners took the ship and left; or died trying...
These are only examples of how a group of humans could find themselves tussling with dinosaurs in a believable struggle for survival/supremacy.
No. Earth is entirely habitable to humans despite the presence of dinosaurs. In fact it wouldn't be out of the realm of possibility that we would bring along dinosaurs if we were setting up a colony. Chickens in particular.