I assume you're talking about the app "Mobile HotSpot" for the n900. That app is only going to work with an ipv4 connection, because it requires an ipv4 address on the GPRS side. Since the gprs ipv6 connection is a /64, you can route that to your wifi and give everyone a public address. I've been meaning to setup a script to do this (along the lines of what Mobile HotSpot does, but for ipv6). I even have radvd compiled for the n900, so automatic address configuration works.