Pettigrew attached himself to James and company because James was the big guy on campus; though I don't think he regularly bullied anybody except Snape, he was smart, he was funny, he was charming, girls liked him (except his target Lily Evans, of course, and she eventually came around), he was Quidditch star, and he was a powerful young wizard. Sirius was much like him, and Remus wound up filling them out by having a more reticent, quiet kid who was, most likely, better at book learning. Pettigrew attached himself to this group because he was hoping for power by proxy.
Thus, when Voldemort began to rise, Pettigrew's true colors shone through. Voldemort was proving to be the most powerful wizard of the age, next to Dumbledore; he could destroy and kill without a qualm, he threw the wizarding world into disarray, he led a campaign of terror and destruction and had it not been for Harry, there is a very real chance that he would have won. Pettigrew was always more of a follower; he just allied himself with the biggest fish in the pond. That was once the Marauders; after school, during the war, it became Voldemort.
My real question is how Pettigrew was sorted into Gryffindor. It seems like it was almost a default position: he was clearly not ambitious (he always chose to work under somebody else), he was not the smartest student (he never struck me as being Ravenclaw brilliant, certainly), and obviously he was not inclined to loyalty. It's like the hat said "Wow, you... wow. Um... Gryffindor?"
Perhaps JKR should have that an interview question, if she hasn't answered it already. Maybe the fen will go berserk and try to figure out how Pettigrew's nature ties in to Snape being the messiah.
Pettigrew must have been fairly intelligent- he managed the animagus transformation. Sure, James and Sirius had to help him a lot, but the fact he managed a difficult spell at such a young age shows he had some brains goin' on.
I've always interpreted the Sorting Hat as basing its decisions more upon what you value than what you actually possess. Someone who wants to be brave, who views courage as the prime virtue, whether or not they were actually terribly courageous at the time or if they were also incredibly intelligent. I wouldn't even call it personal fanon - it's more just a sense I've got, I'm not really sure why.