This isn't comet at all and it is pretty inefficient. The script uses zero of YOUR server resources but some server somewhere is getting absolutely slammed. I made a chat application using this method thta allowd you to share photos as well. Worked reasonably well on a very small scale but it certainly is not comet.

Doesn't look particularly Comet like to me, it's polling the server every 2 seconds for new messages. (Fair bit of bandwidth also).

Interesting though.