Did you ever have a pre-release of iOS 5 on your machine? I'm guessing the version of iOS 5 stored in your iTunes is an expired beta.
No, actually this is the first iPhone I connect to this laptop. Never connected an iPad to it either and haven't connected my iPod touch in almost a year. My iTunes is version 10.5 but I'm still using snow leopard. I don't think that should make a difference though.
If I can't get this fixed by friday, I'm gonna have to take my iPhone back