The first alpha release of Ubuntu 11, dubbed Natty Narwhal, has just been released. It is classed as a "developer snapshot," and is provided more as a sneak preview of what's to come in Ubuntu 11. Regular users of Ubuntu 10 should not upgrade.
Along with the usual package updates and an updated kernel (2.6.37), Ubuntu 11 will also ship with Firefox 4. The alpha build includes Firefox 4 beta 7, and it will be upgraded with newer builds as they become available. The other big change is that the Unity desktop environment now enabled by default, replacing GNOME. You can still switch back to another desktop environment, if you don't like Unity (but looking at the screenshot above, how can you not like it?)
There are various changes and notes for the other Ubuntu releases -- Kubuntu, Xubuntu, Netbook Edition -- but mostly, this alpha is focused on the standard Desktop Edition of Ubuntu.
