updated 12:59 pm EST, Mon February 11, 2013
Downgrade only solution so far
Last week's AirPort/Time Capsule firmware update is breaking IPv6 tunneling for some users, according to complaints on Apple's support forums. Tunneling allows IPv6 traffic to flow through IPv4 links. Without fixed firmware, the glitch could prevent some sites and services from connecting through Apple routers.
Previous AirPort firmware updates have sometimes broken tunneling, making it likely that Apple is already working on a new one to solve the problem. In the meantime, however, the only solution appears to involve downgrading. People can access the option through AirPort Utility; the process works differently in the Mac and Windows versions of the software.