Mork,
There is no direct support for any Linux OS other than Red Hat, only because Red Hat is the OS we have internally to test on. From a personal stand point I think RedHat or Fedora Core can be a bit confusing when trying to run Java apps because they ship 1) natively compiled Eclipse that we don’t support and 2) gcj as the default Java environment, again which is not supported.
If you were moving from Windows and wanted to focus heavily on thigns “Just working” I would encourage you to use Ubuntu (Gnome) or Kubuntu (KDE). I prefer Ubuntu.
If you love playing with Linux and want to obsessively control everything installed on your computer and fine tune the performance of your machine down to a rediculous degree, I’d suggest Gentoo.
If you really want to stay main stream, Fedora Core 4 should be fine, but note the issues above, you will need to work around those (easily) but just making sure to install Sun’s JDK and Eclipse and using the -vm switch to force it to use your installed JDK and not gcj on accident.