According to the release notes, iTunes 9.2 includes the following new features and improvements:
Sync with iPhone 4 to enjoy your favorite music, movies, TV shows, books and more on-the-go
Sync and read books with iPhone or iPod touch with iOS 4 and iBooks 1.1
Organize and sync PDF documents as books. Read PDFs with iBooks 1.1 on iPad and any iPhone or iPod touch with iOS 4
Organize your apps on your iOS 4 home screens into folders using iTunes
Faster back-ups while syncing an iPhone or iPod touch with iOS 4
Album artwork improvements make artwork appear more quickly when exploring your library
Don't install today's iTunes 9.2 if you plan on using Spirit for fresh JB (but it's fine for PwnageTool & redsn0w users)
Since iTunes 9.2 is required to install iOS 4 it looks like the warning is a temporary one and users who want to jailbreak their iPhone will probably have to wait for the iPhone Dev Team to release their jailbreaking tools for iOS 4.
Please note that if you've already jailbroken your iPhone, iPad or iPod Touch using Spirit then you can upgrade to iTunes 9.2. This warning is only applicable for those user who haven't jailbroken their iDevice yet and want to jailbreak using Spirit.
We will also let you know if we’ve any further updates or if we discover something interesting in the new version of iTunes so stay tuned here at iPhone Hacks or follow us on Twitter or subscribe to our RSS feed.