Today's AVCHD cameras can shoot in interlaced or progressive mode. NTSC modes are 60i for interlaced and 24p for progressive. The PAL equivalents are 50i and 25p. Early AVCHD cameras were interlaced only.

Interlacing is the practice of displaying a single frame of video as two 'half' frames. Each frame is split into alternating lines, so that the first frame displays lines 1,3,5 and so on, while the second frame of the pair displays lines 2,4,6 and so on.

For all of the gory detail on why interlacing came about, read this Wikipedia article.

If you look at a raw interlaced clip converted by VoltaicHD for Mac, you will see the interlacing on playback. If you look at a clip from the same camera taken in progressive mode (24p or 25p), there will be no interlacing.

When you look at the same clip under iMovie or Final Cut, you won't see the interlacing. This is because the editor applies a deinterlace filter to the preview window. The underlying clip is still interlaced.

When you export your completed movie from the edit suite, it will be deinterlaced for you automatically by iMovie. If you are using Final Cut you will need to apply this filter manually to your completed sequence timeline.

New AVCHD cameras generally have a progressive scan mode (e.g. 24p on NTSC, 25p on PAL) which is more 'HD friendly'. As the HD workflows mature, these will most probably become the standard.