I'm waiting on a Sena case (taking forever to get here, USPS I guess), but in the interim, I see that PDAir has an Aluminum case out, doesn't look too bad:
PDair Aluminum Metal Case for Motorola Droid Milestone - Open Screen Design (Silver) by PDair - PDair.com

(also comes in black, which might be more in keeping with the Droid original color scheme)
and they have a video demo up:
http://www.youtube.com/watch?v=vVbI1MfAogE
Looks like they handle the sliding keyboard by just opening the case up, right side hinged. As near as I can figure, it has holes in all the right places, although I'm not too sure about the speaker, as that's covered up a bit (some other cases have a smaller opening, maybe that would have been better), but probably still works.
Being Al, I would guess that the protection is going to be pretty good, excepting of course the screen, which given the mounting on the back for the clip, would be pretty exposed if you wore this on a belt. Still, in a pocket, screen inward, probably about as protective as you're going to get.
This is the first metal case I've seen for the Droid, so progress is being made.