I've actually never experienced that problem using my Canon GL2/Sony DSR11 units for digitizing from MiniDV tape. Dunno if the issue has some bearing on use of SDHC media though, if the Z5U is tapeless.
When/If for other reasons I have out-of-sync audio, however, even though it might be a bit of a hassle to do so, you can drag the clip into your timeline go to advanced and extract audio, allowing you to shift/nudge the audio track forward/back until it lines up - depends also on the version of iMovie you might be using, I suppose, never went beyond Ver. 6.1 myself because there were things I didn't want to sacrifice or lose in the newer versions, and I prefer the timeline setup of that version over the current way things work in iMovie - when I need/use it instead of Final Cut Pro for something quick, down and dirty.