I have a similar problem with commandButton. Actions are not called. This affects the 3.0.1 release of RichFaces. The June 1 snapshot build works as expected.
Oh, and as noted elsewhere, there is a compatibility issue with versions of Tomahawk.
For the record, here's what works for me:
Tomahawk Sandbox 1.1.3 snapshot (2006/05/26)
Richafaces 3.0.1 snapshot (2006/06/01)
If I change either RichFaces or Tomahawk to use later releases, then the action on the a4j command button no longer fires.