iMovie '11 AVCHD videos
You can import the raw video through whatever means you like, and utilize Handbrake ([enter link description here][1]) to transcode it to mp4 format, which iMovie can use. This way, you can make use of the H.264 codec to encode your 1080i videos, retaining full quality, and compressing them better than Apple's compression schemes. Personally, however, as one video guy to another, if you're going to deal with a lot of HD, just shell out the cash for a bigger drive, like an external or something. It'll save you headaches in the long run, trust me.

Good luck and happy transcoding!