GTA IV prolly isn't coming to PC. The Bully isn't on PC, Manhunt isn't.. Rockstar seems to slowly be pulling away from the PC ports. I love GTA IV. So f*cking awesome!
Rockstar is a business, businesses are there to make money. With this simple equation you can see how obvious it is that it will be on PC.
Low cost of porting + Sale price of a normal PC game + Large player base that will buy it/rebuy it for the advantages of being a PC game = A shit load of cash that Rockstar will never ignore.
Bully wasn't a huge success, the only real coverage it got was from fundy groups, therefore it wasn't ported to PC, just to some easier to port to consoles.
I'd probably say, 8 months and it's out on PC.