After upgrading to 212.1, I then restarted X/KDE4.3.3. And things that did work took forever, such as launching Konqueror. Things that didn't work took longer. Eventually, I attached gdb to a terminally-delayed amarok, and found that it was stuck in libdns_sd.so.1. After figuring out which package this belonged to, mDNSResponder, I noticed, via eix, that it was recently upgraded. So I masked it, re-ran emerge to update world, bringing me back to 107.6-r5. And now things are almost back to normal. Even amarok starts. This seems like it would block bug #290822, but I'll let an actual dev decide that. I'm going to consider scarabeus' advice and try avahi[+mdnsresponder-compat] instead.
Try the new one... this beast went looong without bumps.