VO and file format

I believe all HD content (HDV or AVCHD) are now converted to AIC/AIFF in an MOV file container. SD HDD and DVD content are converted to all I-frame MPEG-2 video and AIFF audio embedded with start/stop offsets (to reduce audio drift) in an MOV file container.

2. If I have footage of someone talking and want to use just the person's voice in a voice over is that possible?

If the clip is imported to an iMovie/iPhoto Event, then iMovie '08 can extract and "pin" the audio to AV content already in the project. If not, then it can be externally extracted/converted/imported as a separate audio clip and and used in the same manner as any other audio file. It would not, however, technically be the same as a true voiceover recorded in either iMovie or GarageBand from a mic.