Techcrunch reports that YouTube have introduced a feature which allows you to link to a particular part within a video.
To specify a point, append a tag to the end of your video link with the following syntax: “#t=1m45s” (you can change the numbers before the ‘m’ and ’s’ to edit the minutes and seconds, respectively.
Apparently YouTube will also watch for specific mentions of time in video comments and automatically insert a link to the particular spot in the video being talked about.