If you create a btrfs filesystem with Ubuntu 12.04 for example, and then mount it with a 13.04 live USB, you'll see some references to this in the /var/log/syslog (or dmesg output). I forget exactly off the top of my head, but you'll see the system detecting the older format and handling it seamlessly.
Also, I've done this is reverse as well. You can usually still mount a newer btrfs filesystem with an older kernel without trouble. I think for now it's unlikely to break compatibility.