Does such a thing exist?

Regards,
Dave

Yes, and have done for several years.

Without even doing a google, I would suggest starting by looking at Cisco 800 series routers.

[Edited on 2/12/10 by stevebubs]

Hmmm, Cisco 877 £300. Bit steep for something to play with.

Your ISP would also need to support IPv6 addresses - not sure any of them do yet.....