An 18+ rating from the ESRB means Adults Only, meaning people can't buy it. Sony doesn't allow AO games on the PS3 and I'm guessing neither does Microsoft.
Of course, GTA IV will receive a Mature (17+) rating from the ESRB. I'm still gonna get it, and I don't think the clerks at my local Walmart give two shits if a 16 year old is buying an M-rated game.