first connect your computer directly to the internet to your modem to make sure that you are really getting IPv6 from your ISP. if using windows 7, going to network connection manager and opening up your connection should make it look like this.
second, IPv6 addresses beginning with 2001 are tunneled IPv6 addresses if i'm not mistaken(my 6in4 tunnel begins in 2001). what i'm guessing is your ISP is running 6rd and providing autoconfigured tunnels for their customers. if this is the case, look here: http://www.dd-wrt.com/wiki/index.php/IPv6#6rd
third: if my second point is wrong, see if you can find out whether your ISP hands out IPv6 addresses using DHCPv6 or radvd. dd-wrt only supports radvd(DHCPv6 needs optware).
I don't have much experience with native IPv6 but have lots of it with tunneled.