It's for the 360 only.
For now. There is industry talk that it will be offered for the PC version through GFWL, and for free.
And there is talk from PC Gamer that the PC version will have more extra content before it's all said and done.
Microsoft is looking to throw everything into GFWL to buffalo game companies into shackling themselves to it and making them all ****ty.
Well the expansion should be shorter in filesize since you'll need to have the original gta 4 installed, and it'll be more than likely using those files. As for gameplay, expansions are nearly always shorter than the game itself, otherwise they'd just release it as a full game and charge you full whack for it.
Think of it as a giant mod done by the game developers as essentially thats all it is.
Now what would be really funny is if it got delayed because rockstar couldnt get past the copy protection they put in
OMFG ... That's just dumb XD. Making a super tight security on your game and you can't even get passed it yourself ... LOL XD
And how about new weapons, vehicles, abilities, ...?