Thanks for the answers all!

Perhaps I should clarify, I am interested in a desktop/server
operating system, not embedded. Many of the suggestions given
have been very useful and I can see me reading a lot of source
in the next few days, but none are really what I am aiming for.
Retroforth would be ideal if they had a native system for ARM
or some similar chip, but while there is talk of an ARM port
there doesn't seem to be much progress on that front. While
Riscy Pygness is targeted at ARM it seems to be aimed at
umbilical development of embedded systems. Enth seems to
not be active any longer, but the same author has written
Raven, which although not being what I am looking for either
seems very interesting in it's own right.

I'd really like to avoid x86, AFAIK the single stack, irregular
instruction size and complex instruction set all serve to
hinder Forth. On such an architecture the C/*nix way fits much
better. I'm not a hardware guy, so I really need a system that's
already available. I have a GP2X (werty ranted a little about them
some time ago) which would seem a nice platform for what I would
like, but if a project already existed for some other device I'd
be prepared to get one. Reva may be my best bet ATM, they seem to
be well on the way to supporting ARM, I'll check it out. If that
doesn't fit I'll just continue with my own stuff and release
it under an open source license when it's useful.

One final note, I'm not interested in open source because of
the price, more because of the philosophy: speech, not beer.