Since I ordered a new dedicated rootserver, I had problems with my IPv6 setup. Thanks to Niggurath@#debian.de/ircnet and his tips, I finally found the reason for all the mess when I migrated from my old rootserver with Lenny to the new one running Squeeze.
The problem was that in /etc/sysctl.conf there was the following setting on the old server:
Due to a issue in Lenny, this setting had basically no effect on the old server. Sysctl.conf was called before the IPv6 interface went up, so this setting failed or never got active. Meanwhile in Squeeze IPv6 seems to be enabled at an earlier point in the boot process and the forwarding setting will succeed. This in return resulted in my not working IPv6 setup.
When I disable the line in sysctl.conf everything works just fine: the domU honors the Router Advertisments of radvd again and everything is fine again. Sometimes the solution is so simple, you can't find it yourself. :-)