You're right in thinking that iMovie HD doesn't support AVCHD camcorders like the HS100, and in fact does not support 1920x1080 video (or rather, it does - unofficially - so long as you don't use any transitions other than cuts).
As for Premiere, I just looked at the support pages (
Adobe Premiere Pro CS3), and while HDV and DV are mentioned as supported formats, there's no mention of AVCHD, which leads me to suspect it isn't supported.
You could try VoltaicHD to convert your .mts AVCHD clips to .mov (which is supported according to the PP support pages, though it's only a wrapper and I'm not sure whether it will import absolutely any .mov) but it's not an ideal solution. Otherwise, the only solution would be to update to iMovie '09 or Final Cut Express, or a more recent version of PP which supports AVCHD.
You could alo try posting
here, if you haven't already.
Andrew.