Yes it is online for Wii. Here's IGN's impressions of the online in the game:
Bozon said:
Whether you play against friends or random guitar heroes online or locally, the experience is pretty much the same, as you can pair up for face-off, pro face-off, battle mode, or co-op sessions, and rock out to your heart's content ... Online on Wii is a huge plus, and that alone should make this one a no-brainer for music fans, even if you don't have any real friends to rock out with.
