I have been meaning to actually look at the storage differences between signed and unsigned for a few months now, and I have finally remembered to do it! A quick Google search returned just the information I was looking for, so I thought that I would repost the blurb that was worthwhile.

An UNSIGNED INT can go from 0 to 4294967295 (aprox 4 billion). A SIGNED INT , on the other hand, starts at -2147483648 and goes to 2147483648 (aprox 2 billion).

source: http://www.verysimple.com/blog/2006/10/22/mysql-data-type-optimization-tips/

Nick