I'm running the Windows Store version of the game on a Surface Pro and the on-screen keyboard showed up once and has never come up again since. I can keep forcing the native on-screen keyboard by swiping in from the right on the charms menu (tho I have to do it a ton until it shows up).

Any thoughts? Possible an interface option I messed up?

There's an interface option to turn on the touch HUD, on-screen keyboard, or both. It's under Options -> Advanced... -> Interface -> Touchscreen Mode (At least on Android, I don't have a Surface RT or Pro to test this on)