As astonishing as this problem was, I found a very easy fix for it, suggested in the Microsoft Product Feedback Center, if you can believe it. Apparently, a similar issue was already in the beta 1 refresh that came out many months ago, but was never actually solved. I tried to make my voice heard here, maybe it'll be fixed in the future.

Meanwhile, here's what needs to be done to get networking back: Uninstall (or don't install from the start) the "Microsoft Device Emulator version 1.0 Beta 2 - ENU" entry (BTW, anybody in this world know what ENU means? What's the use of acronyms such as this in my installed applications list? Wikipedia gives a 404 when I search for it...) from the Add or Remove Programs control panel. Networking will be back to normal instantly.

Interestingly, in my normal workstation I've used the device emulator in the past and it used to work just fine. No idea what's causing it to fail on the Tablet PC, and some of the other reports in the feedback center don't seem to be using Tablet PCs at all. Oh well :-)