you'd think something like this would've been caught before release, but it turns out that all current versions of osx 10.4 have a byte order bug that causes 6to4 tunnels to be setup with the wrong address. when the machine's ipv4 address is converted to an ipv6 6to4-style address, osx incorrectly reverses the bytes, resulting in a useless address :(
more info at apple's ipv6 list