Obviously some unix/linux
Not obvious to me.
I would choose OS X Server, Windows XP, or maybe Solaris.
Windows and Solaris should have good java support and should be up to the task. OS X is just plain cool
. The main point that all of them are supported and proven. Win XP will give you a lot of options for software tools, the un*x variants have a more common set of tools but it seems to me they lack decent interfaces and are rather archaic. Though OS X will offer both the archaic stuff and perhaps a few things with an interface that isn't out of the 60's-70's.
But don't listen too me. I know nothing about server side stuff