I'm also wondering this as well. Currently I have a 13" MBA (mid 2012), and the lack of a retina screen, while something that never bothered me initially, is beginning to become more and more noticeable when jumping between my iPhone 5s and retina iPad Mini. I recently purchased a DSLR and viewing/editing photos on a non-retina screen is not ideal. I've noticed too that my MBA occasionally stutters when dealing with 1080p video from my GoPro. Basically, I just want something that's a little more refined and powerful (e.g. the rMBP). I like the overall smaller footprint of the 13" rMBP as well.
The only reason I'm waiting is because of the rumors that Apple may release a redesigned 12" retina MBA. If it is super thin and yet still a capable machine, I may be swayed to consider that over the power of a rMBP.
A larger screen would be nice, but I think the 15" would ultimately be too large for traveling and sitting and using comfortably when not at a desk. My ideal setup would be a 13" rMBP and a next-gen Thunderbolt display. Plus the 15" is almost $2,000 and that's just too much to pay for a single machine, especially when it will inevitably be obsolete in a few years. Can anyone confirm the viewing resolution that the current 13" rMBP runs in? I know for awhile they were only running in a viewable 1280 x 800 resolution, which actually provided less screen real estate than what's on the MBA (1440 x 900). I think 1440 x 900 is kind of tiny on a 13" screen, but I've kind of gotten used to it at this point.
