You heard wrong. Unofficially, most devs believe it to mean Internal Operating System. We don't have the official definition, but it makes the most sense.
I preferred whoever voiced Wheatley in the original trailer But still looks amaaaaazing I'm gonna force myself to wait until they release a gold/collector/extended edition sort of thing.
@Xdqwerty, Osu! Tatakae! Ouendan! is the Japanese version of the game, different settings/characters/songs but otherwise identical mechanics. I played that before I knew about Elite Beat Agents lol. Both fantastic games https://en.wikipedia.org/wiki/Osu!_Tatakae!_Ouendan