As far as I can see,  NetBSD is an excellent platform for your project. You can
scale it down as far as you like, it's well ported and technical support is fine
:-)

I think you will find this link interesting, since it's almost what you want
(for cpu and os-platform):

http://crl.research.compaq.com/projects/personalserver/ (if you do not already
know it).

They seem to support the Linux kernel now too, but they used to run with NetBSD
and I think the MIT robot contest still uses the NetBSD kernel. In theory Linux
has one advantage over NetBSD when it comes to Java: that it supports kernel
threads (as cloned processes, but since Unix processes are almost as lightweight
as threads in other OS's, efficiency is fine). Kernel threads (along with a
threaded kernel) enables Java threads to take advantage of an SMP environment.
But in your environment, it should'nt matter at all since I assume you do not
have more than one cpu. You can use 'green' threads (emulated by the JVM) or the
GNU portable Pthreads-implementation.

Linux has one disadvantage in my opinion: though the kernel is well ported to
several architectures (including ARM), the rest of the os (libraries, utilities,
drivers) is not.

Anders