From the rules:
Top 128 from each region at the end of round 1 - Gran Turismo download voucher for PSP
Top 1 nationally at the end each week of round 1 - playseat
Top 5 nationally at the end each week of round 1 - DFGT
Top 20 nationally at the end each week of round 1 - poster
You can't win any prize twice, so 5 different drivers will win a playseat, 25 different drivers will win a DFGT, and 100 different drivers will win a...poster ($10 value). You can win each of the 3 prizes, though. Whoever finishes first will win all of the above, unless they've already won one (or multiple) prizes from a previous week.
Top 16 from each region at the end of round 2 - GT5 Collector's Edition (don't you already own the game if you make it this far...whatever)
Top 8 from each region at the end of round 3 - travel and accommodations to national finals in Orlando
Top 16 from national finals - entrance to GT Academy as long as you meet a bunch of rules (pass a physical, aren't a pro driver already, etc...)