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Venoms have evolved in certain classes of animals on Earth (notably snakes and frogs, but also fish and maybe elsewhere that I can't think of right now), sometimes for defense and sometimes (particularly in snakes) for offense or hunting.
Assuming an evolutionary process, could venoms similarly evolve naturally in mammals? What might be reasons why they might not? (For example, might it be too costly in terms of energy to produce a venom for it to provide enough advantage to be worthwhile?)
Actually there are venomous mammals however they're obviously not widespread. I believe venom could evolve in more mammals but hasn't because of lack of requirement for it.
Consider the primary species which are poisonous or venomous:
Venom and poisons fall into two categories, as a weapon when hunting and as a defense mechanism.
My suggestion would be that what groups all these species is their relatively slow speeds. Many of them are cold blooded and are vulnerable in cooler environments. The poisonous fungi certainly aren't renowned for going anywhere quickly! Even some of the fish (jellyfish for example sting their prey to prevent it escaping while it's digested).
I believe that most poisons and venoms have developed to make up for the creatures' weaknesses.
By contrast mammals are mostly fast and active. They don't suffer in cooler temperatures and generally rely on their speed and senses to escape harm/catch prey (something a nettle rarely does). As such they've never developed a need for venoms.
As to whether more mammals could evolve venomous characteristics I don't see why not. After all it's a competitive world. If a rabbit suddenly developed a venomous bite it would certainly dissuade a cat from hunting it!
I leave you with this thought from XKCD: Snake Venom evolved from saliva, which means that it all started with a snake who's mouth was slightly more gross than usual...
As Liath pointed out, (I just looked that question up yesterday trying to come up with a question to pose to the group.) If you read the article venomous all of the listed mammals are pretty small. The Duck-billed Platypus is the largest but even that is really pretty small.
The Platypus is the only one that has it as a defense only weapon. One of the problems would be generating a toxin that will harm your food/enemies but will not cause adverse effects in the host. Most of the animals listed are rodents and the toxin is mostly for feeding and against invertebrates. The Vampire Bat is technically venomous, but it's venom has an anticoagulant and a local anesthetic, sooo...
Mammals are the biggest animals on the planet so we can take what we want without having to resort to costly poisons. Even the biggest snakes are non-poisonous. I suspect that if we (mammals) evolved with the dinosaurs more of us would have had venom in one form or another.
If a trait is beneficial to an organism, it is more likely to be selected for. You could imagine an environment (resources, predators, etc) where venom could be useful enough for a mammal to provide an evolutionary advantage given enough time. Perhaps if a mammal had to compete with another species for resources, and they were fairly well matched competitively, a mutation (or series of, more likely) could eventually provide an advantage for one or other. Such a mutation could be venom.
You could alternatively develop something between venom (which is injected) and poison (which is ingested or absorbed), like the toxin that a slow loris has. They have glands on their arms that secrete a substance which, when mixed with their saliva, is toxic to other animals. (I forget the details.) Obviously this evolved because it provided an advantage to slow loris; they are small and slow, and it is a useful defense against predators.
It's not a well known fact, but there are actually a species of venomous mammals already out there!
This platypus, renowned as one of the few mammals that lay eggs, also is one of only a few venomous mammals. The males can deliver a mega-sting that causes immediate, excruciating pain, like hundreds of hornet stings, leaving victims incapacitated for weeks.
This is from "Unlocking the mystery of the duck-billed platypus' venom", American Chemical Society. I can't figure out how to link it.
As pointed out by other posters, Venomous mammals do exist and are fairly widespread - even the common shrew could be considered venomous.
Shrews are unusual among mammals in a number of respects. Unlike most mammals, some species of shrews are venomous. Shrew venom is not conducted into the wound by fangs, but by grooves in the teeth. The venom contains various compounds, and the contents of the venom glands of the American short-tailed shrew are sufficient to kill 200 mice by intravenous injection.
Alternatively, lots of mammals have pretty nasty bacteria and digestive enzymes in their saliva, which, whilst not technically 'venom' have pretty nasty cytotoxic effects - leading to nasty occurrences such as necrotising fascitis, and even death in some cases - e.g. Cat Scratch fever.
I've also heard anecdotally from several sources, that the worst bite you can recieve is from a human - the bacteria that live in a persons mouth can lead to some pretty nasty flesh eating diseases!
