Demo's aren't a Rockstar thing really. In fact, only 5 Rockstar games have been given the demo treatment.  However eager Xbox 360 users have been waiting and hopeful of one to appear on Xbox Live Marketplace for some time now. Rockstar have stated a demo for Grand Theft Auto IV would be not "worth the time and effort", which we think everyone can agree with.

They told VideoGamer.com this week that...

"[M]uch of GTA 4 is the driving around and we wouldn't be able to give out the whole city. We would have to give away half the game for free. People would think Liberty City is a tiny city,"

The representative who was talking to VideoGamer.com also went on to say that Rockstar North and the team would rather put in their time and effort into "making the game look great". It's impossible to argue with that, and it's not like a demo is needed to create interest or media attention for the game, plus the release isn't that far away for a demo or the time required for the demo's creation to be worth the effort.

VideoGamer.com [Source]