I'm also fairly new to ffmpegX, but the first thing I would check is to make sure the framerates match. If your original AVI was 15fps and you encoded to 24 or 29.97 fps DVD, your audio will be out of sync. I haven't gone much past this point yet.
While I like ffmpegX, I'm trying to learn the CLI interface for ffmpeg. Is there a book out there about this? I know that most everyting is in the man page or the web, but sometimes it's just easier to have a book in front of you.
Oh yeah, there is another program, I think Uncle Skeleton wrote, called Synchole. It attempts, I believe, to re-sync audio to the video.