Personally? I don't think it will happen within the next 2 years. I don't think we'll see one before 2014, unfortunately, as much as I want one to come, and soon.
The specs of the 15" are nice, but honestly... I don't need super high specs anymore. The only thing that makes me want a 15" MBP is to buy the classic 15" with the screen upgrade to 1680x1050. The specs of the 13" do me fine except the **** screen resolution... 1280x800 is not enough. I had 1440x900 since 2007, and 1680x1050 since 2009. Going back to that resolution would make me want to gouge my eyes out with a rusty spoon.
The current
2012 edition 13" Macbook Pro have the same screen resolution as the
standard white Macbook of 2006.
I just read that to myself and can't stop laughing. That is hilarious. Oh, Apple.
It needs to be upgraded, but my bet is on that it won't be any time soon.
Apple sell millions of the 13" MBP as it is... look at it from their perspective. They sell millions as it is, with the Fisher Price screen resolution of 2006. Why would they bother investing time, money, effort, research etc. in to putting a retina display in the 13" when they're already laughing themselves to the bank?
I don't think Apple are as innovative with their laptops [yes, I know the MBP Retina was just put out... the first "proper" update in what, 5 years?

] anymore as they used to be. Their focus on innovation seems to apply mainly to the iPhone and iPad now... I don't think they care as much for pushing boundaries on their laptops as much as they once did. Hence, I don't see it happening any time soon.