Having worked in the cell phone industry for quite a while now, I'm pleased to say that there exists no 'reverse lookup.' The only way you could find this info is by contacting an unethical/dishonest employee of the carrier which services the number you are looking for.
In the USA, LNP(number portability) adds a frustrating step to someone trying to find out who owns a number. There are quite a few sites out there that tell you which companies own which blocks of numbers, but LNP throws in a big monkey wrench - especially if you have no way of doing an LNP lookup on a number. For example, if you had a number with Cingular from 5 years ago and you wanted to switch to Verizon, you can take that number with you. If you looked up that number on one of those websites mentioned above, you would find that Cingular 'owns' that number block. But in reality, Verizon now services it. If you think someone has Cingular because their number appears to be owned by Cingular, don't count on it. It could be ported.