Priorities are different from procrastination.
If Apple has a perfectly good socket 2011 Mac Pro sitting in the lab by late 2012 there is zero good long term upside to waiting until July 2013 to ship the product. None. zip. nada.
If Apple doesn't ship until post July 2013 it would very likely mean there was no board/product/etc finished until several months into 2013. The simplest explanation for not having one is that they didn't start to make one until late. That's far more likely than the "having trouble with Area 51 alien technology" kinds of explanations that folks have thrown out here.
Or the kinds of explanations where folks invent new products that Apple didn't build as to why the problem shouldn't exist (i.e. if Apple had used last year's iMac case design, it would have shipped on time).
There is nobody else out there shipping laminated screens that large either.