Major Instant Messaging client, IM+, BeejiveIM and AIM now support Push Notification. Push notifications require the 3.0 firmware that was released last week.
Instant messaging apps always lacked this feature on iPhone and iPod Touch. Before release of iPhone 3.0, if we were to switch out of these apps, there was no easy way for the applications to notify you of an incoming message. Although, IM+ provided not-so-Push, which used to send you an email notification of IMs to make the absence of real Push better!