While Shape does have a decent BlackBerry PlayBook Instant Messaging client called IM+ they have been pricing it a bit high at $7.99. This has left space for them to release a free version of IM+ for the PlayBook but there is a catch. This free version is not a free copy of the BlackBerry PlayBook optimized version of IM+ but rather a port of their free Android IM+ app.
This means this free version is not designed for the PlayBook but it does work decently. Check out IM+ free free for the PlayBook in App World.