I've been running windows 2008 64-bit for the past couple of months and haven't had any problems with the framework. At the moment, I'm using the 32-bit version of java, which works fine for me.
Just tested a medium-sized project with postgres 8.3 and that seems to work, but I haven't tested it extensively.
Update: there seem to be two minor issues: One is that the load time of the project will take a little longer (because the project loading driver is 32-bit only). The other is that if you want to use integrated security with SQL server you'll have to replace the sql lib with the 64bit version in the "lib" directory. None of these two issues prohibit you from running it under windows 2008/win64 though.
(note that these opinions are purely my own and based on my personal experience, as far as I know there aren't any official reports on compatibility with windows 2008)