YouTube have added support for deep linking, which means that you can link to a specific point in a video by adding the timestamp to the link. Like so:
http://www.youtube.com/watch?v=AeZxRYXZ154#t=0m39s
By adding #t=0m39s, you’ll send the visitor 0 minutes and 39 seconds into the video. About time huh? (Thanks all who sent this in!)