This is practically a non-post as building wxWidgets with the recently released Visual Studio 11 Beta turned out to be completely uneventful. After installing the Visual Studio itself -- which was also perfectly straightforward albeit still a typically Microsoft experience as, obviously, installing a compiler should require you to reboot your machine -- I could build the current wxWidgets with the new compiler from command line using nmake /f makefile.vc without any problems. I did fix a harmless warning about an unrecognized Visual C++ version but no other changes were needed.
So far I don't have any experience with the new compiler yet, i.e. there are no obvious differences compared with VC10. It seems to produce slightly larger binaries (~70MiB instead of ~68 for VC10 for wxmsw29u_core.lib) but this is perhaps a consequence of its beta nature. I do look forward to testing the improved support for C++11 in the new version and, if I have some time, I'd like to play with the new WinRT stuff and, in particular, WRL library to see if it's realistic to write Metro applications in C++ without using Microsoft-specific compiler extensions and, if so, whether we can support Metro in wxWidgets.