It would take billions of years of utilizing the entire GPU force we have mining bitcoins to do it. I wouldn't count on finding any lost coins anytime soon.
Oh. I see. Well, it would be nice to know the theory though.
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.