Interesting. It's only possible to terminate 2002::/16 using a public IPv4 address, so if you're behind a NAT router, then the router itself must be running 6to4.
Aha! Thanks for the hint: I recently had to reconfigure my router from factory settings. The IPv6 configuration, sure enough, was kicked into 6to4 mode. I set it to "Auto Config" and now I've got end-to-end IPv6 connectivity with, look Ma, no NAT!
