Here is how I understand things to be.
Surface RT does not support Miracast out of the box. However, Tegra 3 supports Miracast, and Windows RT likely supports Wifi Direct (Not 100% sure on this part.) Miracast is implemented through WiFi Direct, so if Windows RT does support WiFi Direct, then
there is no reason it couldn't do Miracast. At this point, a single developer may be able to bake it into an app (assuming the SDK supports this in some way). However, for it to be truly useful, it would have to be baked into the OS (similar to Play To), which
is up to Microsoft.
I believe Surface Pro will support it out of the box due to it having an Intel Core i5 which should support Intel WiDi and Miracast. This support will be provided by Intel and not directly through Microsoft.
tl,dr: Surface RT will not support it until Microsoft bakes support into the OS. Surface Pro should support it out of the box (I think).
Edit: Forgot to mention that Windows has supported WiFi Direct since Windows 7. I'm just not 100% sure it is supported in Windows RT. I assume it is, but I haven't looked at the documentation.