kind of ass backwards, but right now i'm actually take my AVCHD clips, downgrading them to SD, mixing in SonyVegas 7 w/ Ultimate S plug-in, and once everything is done I open the project in Vegas 9, switch all the files to their according HD file, and then render.
First reason for this is that I don't like Vegas's mixing process without UltimateS, which doesn't work on 9. You don't have that maximum control over every last cut and edit like you do when using UltimateS. Either that or I'm missing something. From all I saw, in Vegas 9 it throws all the videos you want mixed down onto 1 track and you can only cut b/t cams, not do any real special effects.
Second reason is that m y PC is too fucking slow to edit AVCHD