I’d say the status quo is more attributed by the influence of Microsoft. They are by far the more strict of the three to how perhipherals work with their console. Believe it or not, this strict adherence is what keeps devs from having controller wars within the confines of their hardware.

We saw this sort of thing when Rock Band came out. The GH2 & GH3 360 controllers worked instantly with Rock Band. Later, we were told by Harmonix that a patch could allow the PS3 such compatability, but Activision decided otherwise. So, realistically, it all comes down to effort. It helps that each side is aware of each other this time around.

I just wouldn’t hold my breathe that the instruments will be cross platform compatible. Especially since 360 controllers are pretty much locked down (via hardware) to only working with the 360.