Going by historical evidence, Apple tends to release changes onto the 15 inch and then bring it to the 13 (and 17 inch) before. I don't think it would take that long to release this product considering they already spent a lot of time on it at WWDC. The most important questions will be about the GPU and CPU and Apple don't tend to go into much detail with that during presentations anyway.
Well, as for the specs, I wouldn't expect anything much different from the current 13" MBP.
I mean, an Ivy Bridge Core i5 2.5 GHz in the base model, and a Core i7 2.9 GHz (dual core as well) in the higher end model. I don't expect quad-core processors and I guess anyone who is expecting them will be disappointed. I think Apple will use dual-core processors due to heating issues and also to keep a consistent pricing strategy in its MBP line.
I also don't expect a dedicated graphics card. According to these rumors (https://www.macrumors.com/2012/10/1...ro-sai-to-carry-unprecedented-battery-design/), the 13" rMBP will probably have an integrated graphics card, and that, of course, is Intel HD 4000. The integrated graphics card will probably handle the 2560x1600 resolution well. It won't be screaming fast for gaming, but that's not the purpose of this laptop anyway.
The Dell XPS One 27 has a 2560x1440 screen and runs on a HD 4000 on the base model, and the performance seems to be just fine: http://www.youtube.com/watch?v=eRXB8XiWwzk
There are also videos of a 27" 2560x1440 Thunderbolt Display running connected to a MacBook Air, which only has the Intel integrated card.
http://www.youtube.com/watch?v=AMGKPv9Va90
http://www.youtube.com/watch?v=_uUbchklSjY
In these videos, the Thunderbolt Display was connected to the MacBook Air and things ran just fine. I didn't notice any lag. Note that all these videos are from late 2011, so these MacBook Airs are running Sandy Bridge processors at most, with an Intel HD 3000 graphics card. The HD 4000 of the Ivy Bridge is far more powerful than the HD 3000, so it should have plenty of horsepower to drive the 2560x1600 retina resolution.
A dedicated graphics card, on the other hand, would consume more battery and would have more heating issues.
Seriously, I wouldn't expect anything different from these specs. I would like to know, though, if Apple will put a 128 GB or a 256 GB SSD in the base model. That's the big question for me in what concerns the specs.