I just bought PS3 320GB version and Gran Turismo 5 for Christmas. I played 2 days without any problems, but suddenly some menus stopped working.
Soon I was not able to play anything, the game just froze to different places, never finished loading for races or did not even go to "arcade" or "gt" mode menus.
I removed the game datas and loaded the updates again. With my 1 MB connection, it took 9 hours to download 12 updates! And what was the result? I did not work.
So I removed the game data again AND also the game save data. Yes, I lost my game progress, but I was not able to play anyway, so...
I called my ISP and upgraded my internet connection to 10 MB.
Then I started the game again, downloaded updates (this time it took 55 minutes!) and finally started the game. The game started and has been running okay for now.
My conclusion is that the game save got corrupted... It would not be too hard to create some kind of check sum etc to see if the game save is valid or not...
So, the solution that might work also for you:
1. delete game data
2. delete game save
3. start game to upload updates
4. start game to create new game save