One of our favorite pleasures is playing Tetris DS online or various Xbox 360 titles over Live at odd hours against players in other regions. Without timezone constraints, we always had a large pool of players, regardless of the hours. Nintendo could have solved the problem with a latency test (assuming one doesn't already exist). Here in North America, there are times when we have less lag playing with South African correspondents than we do with someone just a few states away.
It is currently unknown if the same limitations apply to other regions. We've contacted Nintendo for a statement.