Bound to happen. Question is are we talking a volume of units enough to matter much? I'd guess probably not.
If 1000 people a week more than usual were swapping PS3s for 360s that would be irrelivent unless it was a sustained effect given in the UK alone we're talking about a tiny fraction of much larger install bases.
Really, for this to be a worry you'd need to be looking at many thousands swapping a week - and that seems very unlikly to be the case.
Try to be reasonable... its easier than you think...







