I'm worried that using two displays with different DPIs will create an odd experience. I would love to hear from people who have this setup.
I have a 13" retina MacBook Pro and a 23" external display that I use for web development. It's actually a great setup since it makes it really easy to test how your designs look on both types of displays.
Is it odd to move windows from one screen to the other and have things change from 1x to 2x and vice versa?
When you move a window from one display to the other, you might notice it quickly redraw at 1x or 2x, though it's very brief.
One interesting thing is that windows have to be drawn at either 1x or 2x in their entirety, so if you position a window such that it spans both displays, it'll draw at the resolution of whichever display contains more of the window. It's rare that you ever position a window like that intentionally, though, so I wouldn't call it a problem.
Honestly, the biggest problem you'll have once you get a retina display is that it'll make every other display you use look horrendous :-) Text in particular is an order of magnitude sharper, so I always run anything text-heavy (IDE, email, etc.) on the retina display.