Very cool. I used Cloud9 for an edX course last year. I have to admit I was pretty skeptical of an online IDE, but it worked surprisingly well (it was a Rails project).
Something to note: These workspaces are huge too (compared to all the other Coding in the cloud services). You can use up to 2.5GB of ram and 10GB of disk in each. I work for Cloud9, happy to answer any questions.
From what I can see at the sign up page, it looks like this is for existing/upgrading customers only. Is that correct?
It's for everyone :) If you don't already use Cloud9 simply create a free account then you can upgrade to the new plan.