iPhone won't restore - "Ineligible Build?"

I have a stock iPhone 4 from Verizon, Model number MC676LL running 4.2.10. I am trying to upgrade to iOS 5 but after it backs up my phone, it displays a message saying my iphone can't be restored and it is ineligible for the requested build. Now, I saw this the other day when I was jailbreaking my iPad. The only tweak I made to my system at all was to the hosts file, which I changed back to normal (as far as I know). I successfully jailbroke my iPad. I figure that has something to do with not being able to update my iPhone. I'm on a mac running iTunes 10.5. My iPhone has not been jailbroken at all. If you can help at all, please do. thanks.