[UPDATE 3: Now that Mac OSX v10.10 (Yosemite) has been released, we can report that After Effects is supported on this operating system. See this page for details.]
[UPDATE 2: Apple has released OS X 10.10 Developer Preview 3.1.0, which solves a launch problem with multiple versions of After Effects. See James Landy's recent forum post for more detail.]
[UPDATE: See James Landy's post on this thread for some more details.]
Apple made Mac OSX v10.10 (Yosemite) available as a developer preview build yesterday, and already we are seeing some reports that After Effects and other Adobe applications may have some issues with this operating system.
Now that we have access to the developer preview build, we can begin testing and developing with this new operating system. We can then communicate with Apple about any problems that may have been introduced and how we may go about fixing them.
Last year, when Apple released Mac OS X v10.9 (Mavericks), we released bug-fix updates for After Effects CS6 (11.0) and After Effects CC (12.0) that coincided with the public release of Mavericks.
If you encounter bugs with After Effects and Mac OS X v10.10 (Yosemite)—or any operating system—please let us know with a bug report.