Rockstar have been spotted registering several domain names, all of which are, wait for it, funny fake corporate spoof titles. Some of them include "CashForDeadDreams" and "StopPayingYourMortgage". Seeing as how GTA IV had fake corporations like these, it's very likely that Rockstar are securing the names in order to add them into a new GTA game.
Now of course, none of this directly points to a new GTA game so I can't say that this is concrete proof, but think about it -- unless an employee of Rockstar is hoping to start a mega-corporation called "StopPayingYourMortgage", it's at the very least right to suggest that it might be proof.