It seems that Rock Band has done moderately well (see: awesome) here in the US, so it's only natural that they're going to want to release the game in other parts of the world. We've known for some time that they were going to have a European release (even if it did take a really long time to get there), but this is the first we've heard about it turning Japanese.
Harmonix and MTV have announced that they are partnering with Q Networks in Japan to create a localized version of the game for the Japanese market. The game will not only feature Japanese musicians, but other aspects (I would guess clothing and venues) are also said to be included.
Seeing that the whole concept of playing a plastic guitar and drums in a video game has been around for years in Japan, it will be really interesting to see how Rock Band fares.