2560x1600 is too close to the 2880x1800 that the macbook pro provides.
If the current processor/graphics needs to be overclocked even on the 15" (and it has a top end GPU), and still can't keep up with the user for things like (...scrolling...) what Anand mentioned in his review, then there's no way a dual core computer with no dGPU is going to handle that.
Maybe when Haswell, comes out and the iGPU is 50-60% better, we'll see one, but it's more likely to be a macbook air. IMO they might merge the 13 inch models into a retina macbook (basic consumer model).
Think about it - we can barely play diablo 3 at native res. The 13" can't play it even that well at its current native res, let alone double that.
You will definitely have to run 3-D apps in non-retina (HiDPI) mode. So really, the only real eye-candy is coming from basic tasks like text editing.