Originally thought to be a feature in Grand Theft Auto: San Andreas, the ability to open trunks was misconstrued by a screenshot purporting the feature which ended up being taken from a cutscene.
However, The Official Magazine caught sight of this feature in their demo of GTA IV. When coming up behind a car, the "LB" (L1 on the Sixaxis controller) prompt came on screen in order for Niko to open the trunk.
We don't know how big of a role this feature will play within GTA IV, but we can forsee several situations in which it could come in handy. Smuggling drugs? Check. Stashing weapons? Check. Hiding bodies (dead or alive)? Check.
Rockstar didn't want to discuss the feature at the present time. We'll have more info on GTA IV soon.