MS is positioned well to be the market share leader next gen, but the 50%+ prediction is too sensationalized. The later smacks of an attempt to create artificial controversy and generate volumes of contradictory responses.
It goes without saying that the majority share scenario completely depends upon MS releasing in 2012 rather than 2013. The former scenario has MS competing with Nintendo's launch (a scenario in which I can see MS leading in sales) and the latter counting on the PS4 to completely tank without any inside knowledge of what the new platform is bringing to the market while simultaneously ignoring major factors like pricing and availability.
So my equally bold "prediction" is that no console manufacture will EVER hold more than 50% market share so long as there are at least three legitimate competitors.