I have been using a Cisco 871 router to do most of my IPv6 lab work and I think it is the right choice given the sort of material I am trying to cover. There are however a lot of options out there that are far cheaper and perhaps just as good. I was looking at the OpenWRT site and I think running their kernel release on a Linksys WRT54GS might be the trick. You can find those units for about $30-40 on ebay which is far cheapers then the Cisco 871's which start at about $300 and up. Only problem is that if you have an issue installing the firmware you have a brick - no fun.
I wonder what other hardware folks are using for cheap IPv6 labs. Lots of folks use an old PC and run Zebra or if it will support Windows XP they can run that for a lot of IPv6 services also. Its not like vendors are labeling their gear IPv6 ready in the home/soho market. The OpenWRT stuff seems to run on a lot of hardware platforms so that might be worth a more serious look.