If Apple introduces a 13" retina without discreet graphics, relying only on the HD 4000, the computer will likely be born crippled.
Yes, a 13" MBP can make do with only the HD 4000 as GPU, but without the significantly greater demands of a retina screen. Bear in mind that the 15" RMBP, with discrete graphics, has some capability issues. The next iteration of will probably see a hardware improvement to address specifically this. Meaning the Version A may shortly be viewed in comparison as similarly challenged as the initial MacBook Air was.
Exciting that a 13" RMBP seems near introduction. Yet the wiser course may be patience and allowing those who will not wait to eventually report back. If without discreet graphics, my guess one of their primary complaints will be performance in this regard.
If the rMBP 13" has performance problems with the user interface (UI), it's because it has a slower processor (CPU), not because it lacks a dedicated graphics processor.
The 13" MBP has been able to drive 2560 x 1600 external displays just fine on integrated graphics for years.