I too was having difficulty importing into iMovie 09 from my HF20. I had been successful previously, but this time no amount of plugging and unplugging or restarting in any order would do. The camera's HDD showed up as a drive in the Finder, but (of course) you cannot import those files directly (they are greyed out).
I then tried five (FIVE!) other iMacs with no success and my older Macbook . . . nothing.
Then tonight I went into the clips on the camcorder and deleted a bunch of clips that I had previously imported (and presumably "deleted") but were still listed in the file structure of the camera. They had no data (they had question marks over them) but they were still 'sort of there'. I deleted 45 of these phantom files and returned to my iMac. After plugging in the camera and setting to PC connection on the camera iMovie immediately asked if I wanted to import them . . . after all the frustration I was a little shocked that it worked.
And it continues to work. If it wasn't such a pain I would try to replicate the problem . . . I just can't be bothered.
I hope this helps someone. I was completely dumbfounded.