I had the same problem. Bought new HDR-HC3, put a tape in it that had been partially recorded on my older non HD mini dv sony camcorder. When I play tape through iMovie, I get blue screen, except for parts recorded on new camcorder in hi def.
Here is the solution: In camera mode on the HDR, set the record setting to DV (not HD). Then it will play fine in iMovie. This seems odd, since this setting is for recording and not playback? However, once I set the camera record mode to to DV, it works fine with iMovie. I have not experimented yet with the portion of the tape recorded in hidef on the new camera, but I assume if I have problems, I will just reset the setting back to hidef recording and it will work OK. Hope this helps.