Some of that definitely went on because of lack of competition, but this time MS won't have their one year advantage anymore.
On Idol Master, they definitely did have some sort of deal - one that Namco eventually pulled out of because MS supposedly wanted a bigger cut of the DLC money pie or something like that.
Yea, I definitely don't expect to see a similar situation of Japanese devs placing a load of content exclusively on Xbox. I wouldn't be too surprised to see a couple of exclusive games funded, in the vein of Lost Odyssey or Crimson Dragon though... more likely on the scale of Crimson Dragon than Lost Odyssey this time around.
The Idolm@ster stuff seems a bit strange. I'd think if you were going to tie a franchise up in order to push your console, you'd pick up something that wasn't going to be a niche inside a niche, heh. But then there were a few 360 exclusives that I never understood (for example Virtual On Force).