One more thing: you can play back the raw video clips using a free media player called VLC:
VLC media player for Mac OS X
It's not essential, but sometimes it's nice just to be able to plug in your card reader and watch the files back without having to go through the whole iMovie import process.
If you want to try it out, the AVCHD files are buried deep in the folder structure of the card. The path is: PRIVATE -> AVCHD -> BDMV -> STREAM. Inside the STREAM folder you'll find the video files (with the .mts suffix).
If you find the playback stutters a little, that's pretty normal - AVCHD is highly compressed video which most computers struggle to play back smoothly (and that's one reason iMovie converts it to a different format, easier to edit and play back).
Okay, two more things: when you come to back up your video, it makes sense to keep just the original recordings, as when iMovie converts AVCHD video to Apple Intermediate Codec files, it produces truly massive files which quickly gobble up hard drive space. Make sure that you copy the entire folder structure of the memory stick when archiving, as iMovie looks for this structure when importing, and won't accept the .mts files alone.
Hopefully that makes sense - take from it what you want!