I already posted some details at http://code.google.com/p/ffmpegthumbnailer/issues/detail?id=51 however the verdict was that the issue should be fixed in the process invoking the thumbnailer. Paraphrased:

What steps will reproduce the problem?
1. Procure video file in state of flux (for example, downloaded or edited)
2. Open file manager (thunar) to video file location
3. thumbnailer continually invoked as the video file changes

What is the expected output? What do you see instead?

On such files ffmpegthumbnailer updates the thumbnail, quits, and then is
reinvoked in loop, causing high CPU usage - and an annoying flicker.

Possible Solutions:
  . identify any existing write handles on video files to avoid creating
thumbnails
  . keep track of modification time, either on the thumbnail or the video
file (might not work if options=noatime in fstab)
  . keep an internal database of last modified files and times


products:
extra/thunar 1.0.1-4 (xfce4)
community/thunar-thumbnailers 0.4.1-3 (xfce4-goodies)
community/ffmpegthumbnailer 2.0.0-2

Linux 2.6.32-ARCH #1 SMP PREEMPT Thu Jan 7 22:19:56 UTC 2010 i686 Genuine Intel(R) CPU T2400 @ 1.83GHz GenuineIntel GNU/Linux