Indeed, if you head over to the latest-trunk directory on the Mozilla FTP server, you'll see that 3.0a1 builds are available for Linux, Windows, and Mac OS X. However, this isn't an indication that the Firefox folks have "leaked" a build or pushed up the 3.0 schedule, but that the developers have started the 3.0 trunk for future development. Mozilla developer Mike Beltzner explains the situation on his LiveJournal:
There's a story on Digg about how Firefox 3 is now available for download. While this is absolutely true, it should be noted that the major difference between a firefox-3.0a1 build and a firefox-1.6a1 build from a few days ago is the version number. This change does not reflect an official release of Firefox 3.0 Alpha 1, it reflects the fact that the version number was changed. These are still what we call "trunk" builds, and we offer no guarantees about the stability of the code therein.
You see, perhaps confusingly, Mozilla and Firefox developers are working on three different codebases. There's Firefox 1.5.0.x for stability and security updates to Firefox 1.5, there's Firefox 2 which will be our next release, and then there's work already begun on Firefox 3 which is planned for release in 2007. Those codebases are listed not only in numerical order, but also in order of stability: Firefox 1.5.0.* is rock-solid, Firefox 2 can be a little wonky (it's an alpha!), and we should really have a sign on Firefox 3 that says "Here there be dragons!". In fact, I'm soon going to wrap up bug 308973, which will rename Firefox 3 from "Deer Park" to "Minefield" to reflect the fact that the builds aren't guaranteed to be stable, and are meant for people who like to live on the bleeding edge and don't mind the occasional crash or dataloss bug.
So, 3.0 isn't really quite ready for release just yet. If you're really interested in running the bleeding edge Firefox builds, you might have a look at the alpha builds for 2.0.