Chat apps do not fall into the "core" APIs that use multitasking. That was one of the big selling points to Beejive (and IM+, but I don't use it myself). With Beejive, it operates server-side and pushes new messages via push notifications.
Chat apps can use task completion to maintain a connection to the server for ten minutes as far as I know. Colloquy does this. It makes it really convenient for people that like to send a quick message and switch or leave the app. When they get a response shortly, they can reopen the app and reestablish that connection (or does the connection stay consistent for 10 minutes no matter how many times you go in and out of it?).
That is my understanding at least.