This is speculation, but not exactly a rumor since it comes from a Sony Europe press release. While talking about the Gran Turismo HD Concept for download on day one of the European launch, Sony let slip a hint that more GT goodness will be gracing the consoles soon. Since Sony and Polyphony said they weren't going to add more to the HD Concept demo (if memory serves), this leads one to believe that Gran Turismo 5 is at least going to be demo-worthy by year's end.
From the press release: "A big year for Gran Turismo starts with a free download for PS3 owners. We can't say too much right now, but it's going to be a big year for this title; Gran Turismo HD Concept is just the beginning." So, maybe Sony hasn't been lying around complacently after releasing that concept demo -- they've been hard at work getting things done. Sweet. What do you guys think? A new HD Concept demo, or GT5?