IPv6 via GPRS is not supported by this package. I do not have IPv6 mobile provider, so I cannot test it. But I can push project https://code.google.com/p/n900ipv6/ to Extras-Devel and make ipv6-support package to depends on n900ipv6 (as ipv6-support is metapackage).

Also now only dhcp (v4 and v6) newtworks are supported. This is restriction by closed source nokia icd ipv6 package. There is also static ipv6 address configuration, but I did not find gconf keys used for it.

Other problems (related to icd) is hard to fix because both icd modules (ipv4, ipv6) are closed and also icd2 daemon is closed...