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).