I believe every single one of the 676 two letter combinations has been
registered already, although you might be able to purchase them from someone
for a lot of money.
here is a listing of most:
http://blog.citycynic.com/2letter.php
a few missing there, if you do a whois on them you find out they are already
taken as well, although I didnt check all the missing ones.
Not sure about letter + number, might want to try that...although again,
theres only 260 of those (2 characters) so more likely than not, they are
all already reserved, probably all by people trying to sell them back by
some obscene amount.
BUT, i just went to my registrar (godaddy.com) and if you try putting in
anything that's less than three characters it gives you a problem....so
perhaps there is so other limitation/rule that is not explicitly described?
Could be that every single possible combination of 2 characters (letters and
numbers) is taken, as well as single letter ones, so rather than do a search
it automatically just makes you enter 3.....not sure, but I am curious now.
-enhypnion