I'd concur with Lukewarmwinner - the rMBP will come at a premium over the current MBPro and so I'm guessing around the same $ as a fully specced up MBA.
only difference would be the retina display and shape/weight.
most seem think it will have integrated graphics only.
that said, one's a 'consumer' Mac and the other a 'Pro' aimed at professionals so that has to be a factor.
the CPU in the MBP will have more oomph and the memory seems to be different from the Air also, though whether Low voltage memory is any slower than regular 1600 mhz memory, I wouldn't know.
by going for a new release Mac, that would in itself be an advantage going forward.
my $2 worth