Michael Coleman is tracking the status of After Effects on Snow Leopard on his blog.
But there's one issue that I wanted to make sure that everyone saw:
After Effects CS4 (9.0.0) on Snow Leopard doesn't work well with Render Multiple Frames Simultaneously multiprocessing. You must update to After Effects CS4 (9.0.2) if you are going to use Render Multiple Frames Simultaneously multiprocessing on Snow Leopard.
Of course, there are so many other reasons to update that you've already done it, right? Right?
Note that you need to deactivate CS4 applications (or just one CS4 application if you are using a suite) before upgrading to Snow Leopard, as you'll need to reactivate after the upgrade (the OS version is somehow linked to the activation).
If you already have used all your reactivation credits (you get two), you'll need to call Adobe support. When you get through, getting a new activation credit is a matter of seconds.
To be on the safe side: better deactivate before upgrading!
Joost van der Hoeven