Sony US boss (talking to that GTTV guy) said something like - Agent is still in production, but most likely is not exclusive to PS3 anymore. I think Xbox360 version is nearly certain, Wii (on) U possible and no chance for PC version. But you're right, no offical word from Rockstar on any possible new games - no new GTA, Bully 2, Red Dead Redemption sequel, Agent...
No way it's coming to 360, Sony and R* had an exclusive deal. Unless you care to provide me with a link where they Sony boss mentioned that.
IIRC wasn't Agent part of the deal for originally Sony-funded LA Noire going multiplat? Maybe Rockstar just bought themselves out of that contract with cash money...