At Google I/O this week, there have been a couple of Comet related announcements.
Firstly, the Google Feed API is now live. The API allows you to pull in RSS/Atom based feeds, and now it does it in real time, well, as long as the feed is using PubSubHubbub on the server side.
Secondly, Google App Engine was always restricted in how long requests could run for, making Comet not that viable. They have now announced a Channel API. Details are thin so far, but this is some form of Comet running on Google App Engine.
It will be interesting to take a look at these when they are available and see what techniques are being used.