Adobe guys are tracking After Effects and Snow Leopard, the 10.6 version of Mac OS X. Besides upgrading to AE 9.02 for multiprocessing, there's not much up. For Photoshop & other products, John Nack is a good source of information.
As a precaution, deactivating AE (or any CS4 app if you have a bundle) might be a good idea since you could avoid dealing with customer service if there's a problem with the OS install.
[update: it turns out that Snow Leopard requires 9.0.2, and you do lose an activation credit so deactivating is advised]
Only time will tell if the QuickTime gamma issue is really solved with current versions of the QT players and other software and old files, etc. While it doesn't get too specific, there's even a Compatibility Wiki by Wikidot for many Mac apps.
Update: CDM has an initial but in-depth review of something new in Snow Leopard -- Mac OS X 10.6: Quartz Composer 4.0 Hands-On Review, New Features.
Update: See the Mac OS X 10.6 Snow Leopard: the Ars Technica review by John Siracusa -- QuickTime is dealt with on page 6 and page 16 of like 23 (via johnmontfx). Chris Meyer has more in Snow Leopard: Hopes, Misunderstandings, and Gotchas, as does Studio Daily in What exactly is this QuickTime X thing anyway?