Welcome to geekhack.
The only other readily available, modern UNIX board with USB that I am aware of right now is the Happy Hacking Keyboard (HHKB). It's a bit on the pricey side, though.
Having gone through several Sun Type 6 keyboards, I got fed up with spending $35 every 6-8 months, and just bought a HHKB.
If you're happy with the Sun though, and want something comparably priced that feels roughly similar (at least, to me it did), there's the HHKB Lite. There are 2 main differences: dedicated arrows keys and rubber dome membrane, rather than topre capacitive switches.
The Lite feels like the Sun, to me, but is significantly more compact. It has an identical layout, within the constraints of its form factor, as well.
The other option for a USB native keyboard is a custom layout Unicomp Customizer. Mine is PS/2, as the first guy I spoke to said it had to be, though I was later told by someone else that it can also be USB. The fee for customization was $20, when I did it.
You could also get a decent USB adapter for a Model M, and do a bolt mod to get a UNIX layout, as you can see in my sig. It's fairly easy to add the remaps necessary to windows. And if you use Linux, I can hook you up with an xmodmap configuration file that does the same remapping, and also adds the HHKB style 3rd layer functionality, using the right "control" key as FN (though I have considered splitting my right shift into 2 keys, and using the rightmost of those to act as a FN key).
So those are the options I'm aware of. There may be others, as well. If you want to go the Model M route, let me know, as I have several already modified to that layout.
