I would say exorcise caution if you attempt this, but they claim this will fix the problem for those of you with "ghost" cars in your garage that cannot start version 2.10. (AND BACK UP YOUR SAVE FILE BEFORE YOU ATTEMPT, just in case!)
Page text: (Pasted from GT Planet)
As many of you have probably heard by now, since the latest 2.10 update for Gran Turismo 5 went live last week some users have been reporting a strange game-disrupting issue involving the GT5 logo screen loop where the root cause was, of all things, vehicles in the game.
Today, the problem has been resolved at the hands of our very own members here [GT Planet] so if you’ve had this problem follow the steps outlined below and you’ll be playing GT5 again in no time:
Tutorial to fix problem with 2.10 update.
1. Downgrade to 2.09 – go to the Game Data Utility and select GT5, hit triangle, select delete in the right-hand side menu. You now have to re-install updates to get you back to version 2.09.
2. You need to find your ghost cars; the cars cannot be chosen, driven, sold or deleted; this does not include DLC cars. When you’ve found them proceed to the next step.
3. Go to your GT world menu and select “My Home Settings” select “Design Options” and change the background color to the original blue and your background image to the first as well.
4. Again go to the Game Data Utility and delete the game to go back to 1.00 version (if version 1.00 does not work for you you could try the first update which is 608 mb and that takes you to version 1.05). You’ll need to use the first edition GT5 disc as no other versions of the disc will get you back to version 1.00.
5. Start GT5 and decline the updates, go to your garage and delete the ghost cars.
6. Once you have done the above steps you can then install the updates bringing you back to 2.10.
After you’ve successfully completed all of the updates and you now have a working save with the latest 2.10 update, you may have to go through the process of reacquiring any or all of the DLC you may have purchased over time, and you can do this by going intoAccount Management on the XMB (under Playstation Network) and download them again and re-install them.
You may notice that once you’re back in version 2.10 you may have a garage full of cars but the thumbnails are now corrupted, to resolve that just hit triangle in garage view and select “Create Thumbnails for All Cars”, this will take time depending on how many cars are in your garage.
When going back to 1.00 or any older version with newer saves (DLC cars in it), be sure to change the saves options settings beforehand to default like in 1.0. Your saves might have difficulty loading in early versions due to conflicting options settings (there are many options that are only available in later versions). Also make sure the save has you using a car that is in 1.0, and turn off all favorites car (if they are not in older version). If in doubt while in version 2.09 buy a car from the new car dealer.