It turns out that Snow Leopard requires After Effects 9.0.2, and you do lose an activation credit so deactivating is advised.
For Photoshop & other products, John Nack is a good source of information on updates.
Updating from the earlier post Snow Leopard, AE, QT, and Quartz Composer 4, 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 a good overview in Snow Leopard: Hopes, Misunderstandings, and Gotchas.
Update: Todd Kopriva notes plug-ins and Snow Leopard: Toolfarm provides compatibility table.