GTPlanet recently did a bit of internet sleuthing into the Gran Turismo 5 official website, and uncovered logos for 24 tracks which aren't currently available in the game. A few have appeared in past titles, including Gran Turismo 4 and Gran Turismo PSP -- but all 24 have never appeared together in the same title. Check out the process through which these logos were divined on this GTPlanet forum post, and check out the full list of tracks after the jump.
It's unclear whether these logos are actually indicative of a batch of tracks that are coming to Gran Turismo 5 as DLC -- though it's worth noting that the list contains the track "El Capitan," which was shown as a GT5 track in a cross-promotional video posted (briefly) by Red Bull last year. If that particular track actually ends up in the game, that would mean that Polyphony Digital has been working on it for a while! Why, that doesn't sound like them at all.