Have you upgraded to Leopard? When you plug your iPhone into your upgraded computer, does iTunes flat-out ignore it?
There is a solution to making the oh-so-ignorant iTunes recognize and sync your iPhone. All you have to do is re-install iTunes.
There is no need to un-install the program; just update iTunes by downloading the latest version from Apple. (Note: if you are already running the latest version of iTunes while experiencing this problem, re-install that version.)
Do that, and your iTunes and iPhone should be syncing like nothing ever happened.
Source: Apple Support Forums