Here's a patch for correctly-rounded round in trunk.  This patch doesn't 
change the rounding behaviour between 2.6 and 2.7:  it's still doing 
round-half-away-from-zero instead of round-half-even.  It was necessary to 
detect and treat halfway cases specially to make this work.  Removing this 
special case code would be easy, so we can decide later whether it's worth 
changing round to do round-half-to-even for 2.7.

I want to let this sit for a couple of days before I apply it.