How to ask for activity (new messages available?) in a chat room?

Hello!

Is there any way to ask for “new activity” in a chat room (using AJAX)? In our special case the user can be registered in many chat rooms at the same time but only one chat room is visible at once. When a message is send in an currently not opened chat room, the name of the chat room should be highlighted in order to inform the user that a new message was send. He then can switch to that room (that reloads the whole page and opens the room in question). Is there any way to ask for new questions since a given date/time?

Thanks for your answers in advance!

Lars