Back in fall 2010, a few weeks before GT5 was released, when Kaz said they were pressing the discs he also said that they were already working on GT6.
I doubt they'll release any info about GT6 apart from footage and/or screencaps from the alpha at any time this year, but they are already developing it. It will have been over a year and a half since that statement by the time this year's E3 rolls around, so I'm pretty sure they'll make some sort of announcement, but we'll have to wait and see until then.