Bitcoin addresses do not get "reserved" when they are generated, it is just astronomically unlikely to generate one that has been used before. In theory, you could keep generating new addresses until you hit one that has coins in it, but as SgtSpike said this would be much more difficult than finding valid blocks.
Huh... So, technically, it is possible to get into any ones wallet like that? That's fun)
Technically, yes. Reality, no.
I think someone mentioned in another thread that the possibility of finding a previously used address with running the calculations on a 5870 was equal to the possibility of a person winning the lottery 100 days in a row. Basically, it will NEVER happen.