Some of Apple's new products which are due in early July suggest we'll see two updates by then, one for Mac OS X and another for iTunes.
MobileMe's terms of service suggest that you need Mac OS X 10.5.4 as well as iTunes 7.7. Meanwhile, the system requirements for the iPhone 3G also list iTunes 7.7 as a requirement.
The current version of Mac OS X is 10.5.3 and was released only two weeks ago while iTunes is at version 7.6.