AFAIK there is no official policy on user names, and the only one on avatars is that they should be PG-rated (i.e., nothing offensive).
Users on SO aren't allowed to solicit other users, so if a fake Jon Skeet were trying to pull a con on another user in order to get something, that would be reason enough for action.
Same goes if they were impersonating for the purpose of trolling. Trolling isn't acceptable here, so they can be dealt with.
Impersonating another person can be considered abusive of the community, and therefore should not be allowed. But, at this point, we enter a grey area. How do we verify a user is who they claim to be IRL? Unfortunately, this would seem to be a task that simple moderators would not be able to perform. If it is pretty obvious, that's one thing. But if the user seems like they are the real person, but there is doubt by members of the community, it would require somewhat extended contact with the user to make that determination. And I don't think people like Jobs or Gates would want to waste their time convincing some mod they are who they say they are.
If a fanboi is using a famous person's name and image on their profile, and they state in their profile that they are not the person in question, then I think it should be okay. Obvioiusly, if the real person were to arrive, I think they'd probably be given their actual identity on the site (again, not something for simple mods to do). But until that point, this is the interwebtubes after all. Most of the user names in here aren't real, and expecting them to be so is so foolish as I would hope our user base wouldn't fall for.