According to Vadim at Percona:
It's 100% backwards-compatible with standard InnoDB, so you can use it as a drop-in replacement in your current environment. It is designed to scale better on modern hardware, and includes a variety of other features useful in high performance environments.
The release is pure GPL (v2) and commercial support is available from Percona. If percona keeps this up, they just might become the new MySQL.
The source is available from Launchpad and from Percona. Binaries are also available and OurDelta will start using XtraDB in future builds. Percona expects a 6 month development cycle. I don't see on here how they plan to incorporate contributions but that may just need a little time to figure out. They do say that they have already incorporated most patches that are available and that make sense for their customer base..
This announcement excites me much more than Drizzle ever did.