well, US reg's can only have the last two characters alpha-numeric...
so I have seen N-45EX (a Lancair III)
and to the best of my knowlage N-245EX has been taken, I think N-345EX is still good. (If you can't follow this, write it out in block letters, and think for a minute). Anyththing that ended in foxtrot-uniform would be cool too
Come to think of it, N-1 wouldn't be too bad either (FAA's G-IV, saw her land at Oshkosh last year. "Hold, I'm sorry center, we don't DO holds, just give us direct)