The HD4000 runs surprisingly well at HiDPI res, actually. Haven't experienced any lag myself, and when needed the integrated nvidia GPU kicks in automatically without a hitch.
Well, Sandy Bridge and Ivy Bridge integrated graphics is OK for most users (i.e. not for gamers and engineers, but for most people they are more than they need).
The new macbook pro retina 13" uses only the intel 4000.
I picked one up last weekend and it's surprisingly quick, at least whilst playing with things like WebGL and three.js.