There are many discussion about iTunes 9.0 all over the Interweb so I will not do it again here.
Along with the new iTunes Apple had released the iPhone firmware 3.1 (technically 3.1.1) for the iPhone and iPod Touch. In doing so, Apple also reduced the cost of upgrading the iPod Touch to USD4.95. For those who held out when the iPhone firmware 3.0 came out, there are even more reasons to get it now.
The iPhone users who have jailbroken or unlocked their iPhone using unofficial methods should stay away from both iTunes 9 and the iPhone firmware 3.1, at least until the iPhone Dev Team releases new versions of their software. Accidentally upgrading your iTunes to 9 and more importantly upgrading your iPhone to firmware 3.1 may prevent you from ever unlocking the iPhone accept via Apple’s official methods.