Well if one were to look at Capcom's approach, as well as the guys behind Burnout: Paradise, then making multi-platform games doesn't seem to be troublesome. CoD4 also seems like a good valid point.
I just think it all depends with the minds working at said developers. One developer may know how to work their magic on one console, and find the other to be a "stonewall," whereas another developer can make both versions both shine.
I heard that making games on the PS3 first and then porting them on the 360 is actually cheaper, and I don't know if this is true, but if it is... WTF man? Maybe it does depend on the developer.