This has been discussed numerous times in the Tracks Master List thread.
Personally I believe that most, if not all, GTPSP/GT4 tracks will appear in GT5.
There are too many of them that can't NOT appear in GT5 (with the exception of GT4 tracks George V Paris and Hong Kong that were possibly the worst in the game - they weren't in GTPSP after all).
The confirmed GT5 tracks and GTPSP (+GT4) together make about 45 unique tracks.
Including all unique variations, that makes around 70 layouts.
Counting in reverse versions too, that's around 110 variations.
Even if we take out some GTPSP tracks, there still is room for additions from NASCAR (very likely, as 2 ovals are too few) and WRC and probably a couple of completely new or returning (from old games) original tracks that we haven't seen yet.
So in the end, in my opinion:
~45 tracks, ~70 unique layouts (courses) - What will be most likely included in the game
~70 tracks + variations - unlikely, but would be great
~?? tracks + 70 unique variations of them - would result in an enormous amount of tracks, I can't see this happen