While browsing around, I found this:
http://asia.playstation.com/eng_hk/index.php?q=node/440 on the Hong Kong PlayStation site. It says:
"Dear players of "Gran Turismo HD Concept":
We have released "Gran Turismo HD Concept" Ver.1.1.
The "forced finish" bug※ has been fixed in Ver.1.1. "Ranking" results recorded by Ver.1.1 players will be given priority in the online ranking for this software. If you are playing an earlier version of this software, it is highly recommended that you download Ver.1.1 and install it.
※The "forced finish" bug:
In the Time Trial mode, there existed an extremely rare bug in which the lap would be abruptly ended while the car was still driving. This would result in the player's being unable to save best lap replay data for that session, and an incorrect time and replay being entered into the online rankings.
We apologize for the inconvenience."
From my understanding,that means the leaderboards were updated in response to the glitch where people were completing laps on the second-to-last check-point.
There's no mention of this (as of this posting) on the US PlayStation site, but I don't think Americans need to re-download the demo, as it's possible to rank on the leader board, even though previous scores were cleared.