Rockstar's outlined the DRM methods used for the PC version of GTA IV in this IGN article, confirming that you'll need to register the game every time you install it on a new machine.

The title will use SecuROM, but you'll be able to install it as many times as you like.

There's a lot of info through there, so if getting het up about anti-piracy measure is your thing, it's Christmas come early.