Valve have already announced that there will be some more Portal 2 DLC arriving some time early 2012. Not too much is known yet, prices and what it will fully contain have yet to be announced, as is whether or not it will be available on all platforms. What is known though is that people will be able to create their own test chambers!
How fucking cool is that! In a sort of Halo Forge style you will be able to create your own test chambers post them online and people will vote for the best ones. Now that on it's own actually sounds like the shiznuts so I wouldn't be surprised if end up getting it even if it isn't free!