Generating random unique alphanumeric IDs

Does anyone know a way to generate a random and unique lowercase
alphanumeric ID (preferably without using 0, 1, o or i to prevent problems
with users manually typing the ID) using SQL without resorting to a
prerendered table or using GUIDs.

For example, if I were to ask for an ID of 5 characters, something like the
following would be returned:

hn21o
8sp2j
9wwun
m7z02

Notice that I don't mean hexadecimal values either. This would preferrably
not resort to trying to generate the ID, then checking for a clash, and if
there is one, do it again, although that could do as I can't think of how
the ideal solution of a ID hashing algorithm would be possible.

Any ideas?
Thanks

Thom