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Adobe released an update ("moviestar") to Flash 9 on January 28 which now includes an H.264 codec. (Was previously in beta since August.)
Of course, it will take time for the new version to gain traction. But I'd expect YouTube and the other big sites to support it quickly, probably in a backward-compatible fashion. (Use the best codec available in your Flash player.)
Add &fmt=6 on the end of a YouTube video URL to get H.264 (only), or &fmt=18 to get either H.264 or the old format if not available. (&fmt=6 will just not load the video if it isn't available in the new format.)
This is apparently using the new Flash H.264 support, so you may need to update your Flash player.