It all depends on your environment. No doubt some find cutting code in XCode with multiple simulators and IB open might be a little constrained (due to screen size more than anything), but actually writing the code should be more than fine. If you're a full-time iOS dev and an ultra-portable laptop is your only machine you're probably a little crazy anyway.
Personally: I've already written a couple of solid libraries for Go on my rMB and enjoy it. The screen is great and at 1536x960 there's more than enough real estate to have multiple files open at once, or half-docs/half-editor. Just like any other modern laptop.