Firefox has as full DataChannel implementation (SCTP-based), while Chrome has a partial one (RTP-based). They don't interoperate yet. I implemented most of the partial implementation and am working on the full implementation now.
I'm glad that you're excited about them. I am too. I hope to have it ready for you soon :).
Well, when I heard that WebRTC was in Chrome stable I thought this must surely include DataChannel, since it's simpler than a full-blown video stack. So although it being behind a flag is better news than it not being there at all, I was still pretty disappointed when I read that page :)
edit: also, I don't see it as an about:flags flag.
edit 2: I should just tunnel data through video then, what could possibly go wrong?
It's not quite that simple. We (I work with the Chrome team) have an RTP-based data channel (sort of what you call "tunnel through the video" already in Chrome), but it provides no congestion control or reliability. To provide those, the IETF has proposed as the standard to use SCTP. Firefox has already integrated SCTP into their implementation, and we're working to get it into Chrome. So, you can start using the API now, but it'll get much better soon.