Instead of client/server, why not try a master/slave relationship where the master will push the content out to each connected slave?

Is that too much of a simplification?

update: Another thought, even tho' the infrastructure may be trickier to set up, it would make the content management easier, as the master just pushes out all the latest content (every n seconds), rather than the slave requesting the newest content it hasnt recieved yet. On the other hand, under load there would be noticable lag between the 1st slave served and the last slaved served..

Back to the thinking board.

If each chat line was indexed with a sequence, the slave would then request each bit of additional content greater than the last bit recieved, less some time out period, so they dont get 5 weeks worth of chat downloaded to them.