I happened to find out that the official USB arduino board (which I bought from sparkfun.com) can fit into serpac A-21 box (with a 9V battery compartment).
It's a very tight fit and the screw holes don't align. But with a front panel properly drilled, that little box can be a secure housing for the wonderful arduino board.
On the other hand, if I can suggest that the official version had its board resized to better fit in such plastic packages.
I modified this enclosure from PacTec. Fifteen minutes with a Dremel tool and it fits nicely. My work area is always very messy and the Arduino is now safe from other activities, components, but mostly beer spills.
Looks awesome, sure beats having the board laying out in the open. I bet if you bought 10,000 (one for each of us), you'd get them at a reasonable discount.