We’ve been working a lot with the new H.264 support in Flash. Like the On2.VP6 addition back in Flash 8, the inclusion of H.264 opens up new potential for delivering high-quality video at low bitrates. One drawback is that Flash doesn’t currently support transparency in H.264 – you’ll have to stick to VP6 for your walk-on hosts and so forth.
The latest figures from Adobe put the H.264-enabled version (9.0.115) at 62% penetration for the US and Canada as of March. This is probably into the 80s by now, as the 99.4% of users with Flash 6+ get prompted to auto-update when they encounter Flash 9 content. There’s little reason to hold off from distributing in H.264 at this point: very few users will have trouble viewing it and the quality boost and/or bandwidth savings are significant.