Apple has actually done a really good job of pushing IPv6. For years their devices have supported it from their routers to their laptops. Between Apple and Comcast's recent IPv6 turnup I'm actually on IPv6 by default.
IPv6 works without problems in OS X, but AirPort doesn't support IPv6 over PPPoE (I've had no luck with bridge mode either, even though it theoretically should support that).
I've used a Time Capsule just fine in bridge mode when cabled to a CPE sending out router advertisements.
If you use it as a bridge, you might want to make sure the TC firewall is disabled (or at least lets multicasts through).