Longhorn Server Beta 3 is now officially out. If you are in the LHS beta program, you can download the build from Connect, or otherwise you can get an evaluation edition and documentation from the Microsoft website

Note: there seems to be a bit of an issue with the public download files at the time I'm posting this. If you are getting a 404, then remove the .lnk extension from the URL (so that the file ends in .iso rather than .iso.lnk)