I would advise you to do your building on a build server that has plenty of CPUs, cache, disk, and bandwidth, and just do your testing on the tablet. While it's possible to do development in the tablet environment, the hardware has generally been tuned for multimedia consumption and you will get frustrated using it for stuff better done elsewhere.

You should be able to use the tablet as front-end for your build servers, though. No reason you can't remote through it.

Is it known yet how well the Surface (RT or Pro) works as a laptop - i.e., on your lap? It's unclear if the hinge is rigid like on a normal laptop, or would only stand up using the back-stand - and if the latter, whether that would work well on your lap.

If not, well, there are other upcoming Windows 8 tablet/laptop hybrids that will work fine as a traditional laptop. I'm more interested in the idea of "ultra-portable PC" than "tablet with optional keyboard" aspect of it, so this is something I'm curious about.

I doubt it can work well on lap with the keyboard.
However, that's not a big problem, at least for me.
I use a computing device in two ways, on the desk, or hold in hand and I lie in the sofa. Surface would work quite well for both.