Not much Info floating round as to release date for it, suspecting 2010 only as Kazunori Yamauchi is looking to include major brands like Porsche and so on.
He wants to include damaging, day night transition and weather effects. Remember Polyphony Digital is a very small company, if I recall they were about 63 staff members for GT3
could be more by now but I'm sure it's still small compared to others.
Meanwhile GT5: Prologue will keep expanding via updates so watch out for that, 3 new cars already planned, if not released yet.
Thats all I can remember from reading an interview a few months back.
In between they design special builds of GT for car manufacturers, they also done the console of the new Skyline GTR, so they're pretty busy.