Hi muppetchef,
From experience, this is what I would suggest:
1) Shoot in PF25. I would do this anyway regardless of the specifc issues with iMovie, as I don't want to deal with interlacing at all. The only issue is that with fast moving content (eg sports) it may appear less smoth. You should remember that movies are shot at 24 fps, so if you don't have a problem with the smoothness in movies, I don't think you will find this an issue.
2) If your camera is like mine, when the 25p footage is converted to AIC on import into iMove it will be incorrectly tagged as interlaced. This is only a problem is you apply a special effect that alters frame size (ie. Stabilize, PIP, Crop, Ken Burns effect, Wipes, Ripple, Zoom etc). In this situation, iMovie deinterlaces the content from 1080i to 540p and you lose resolution.
3) To overcome the issue you could try using JES Extensifier to retag as progressive. However, I am not sure how effective this actually is (refer to the discussion in my link in the previous post). You could do some tests yourself by viewing the output on the HDTV.
4) Many people claim that unless you have a high end camera with a large sensor that there is very little vissible difference between 1080 and 540 ersolutions even when viewed on a HDTV. If this is correct, maybe you should be stressed about the issue in point 2 above, or perhaps you can just import at 540 and forget about all these issues. The advantage of this is that your file sizes will be nore manageable, and editing will be less demanding on your system.
5) Regardless of the approach you take, you should ALWAYS still archive your original .mts files through the archive import option in iMovie, so at least you know you have the full resolution native files if you want to redit the files in the future.
6) As a left field idea, you could try using Adobe Premie Elements 10 (just released). This edits the AVCHD footage natively so you avoid all these issues. You can download a trial.
I think the most important thing is to just decide on an approach and run with it. I wasted so much time a couple of years ago trying to sort this out, and it really destroyed my enjoyment of my camera.
Good luck!