At Sony's second press conference at the Tokyo Game Show today (yes there was more to come after the first rather lacking performance) it was announced that Gran Turismo 5 will be released in Japan in March, 2010.
Unfortunately series creator, Kazunori Yamauchi, didn't mention a possible release window for any other region. Interestingly, the official TGS magazine lists GT5 as being only 65% completed. By contrast God of War III is listed as being 60% into its development period, and Final Fantasy XIII as being 90% done.