Paladin is right. The screens on Nintendo of Japan's site suggest that Mario 64 DS has been changed significantly for this release. New mini-games, four different characters (apparently with unique moves) to play through the main game with, noticeably improved graphics, and maybe even a new boss are shown in the screens. Even if they've shown off 100% of the changes in the screens alone, the game is reasonably improved. All that remains to be seen is how well it plays.
The system does need more first party games at/near launch: I probably won't buy one right away unless the reviews are astronomical. But I will buy one when some of the other delicious games that I saw on Nintendo of Japan's site come out. I think it will be a reasonable trade-off if Nintendo gets a significant head-start on PSP and gives us a consistent selection of new games instead of releasing a huge pile followed by nothing until next Christmas.
Edit: fixed miscellaneous errors.