iMovie 09 automatically converts AVCHD to AIC, a much larger file size, for editing. BUT THERE IS A CATCH. If you apply any special effects other than color grading (and a few others) it converts the entire file to 580p. Even if you output it at 720p for YouTube it is still upsizing a 580p image. There is an utility (iMakeFullHD) that you can use to mark segments with no effects, and thereby prevent iMovie from downgrading those shots to 580p. It works only in Leopard. Having said all that most projects to YouTube, even HD projects, seem to be ok at 580p, just not the best they can be.