If there's interest I'll hack up a FreeBSD nanobsd image with ipv6 support, a routing daemon (whatever people think is good enough) and whatever other stuff is "enough" to act as a 6to4 gateway. You too can build diskless core2duo software routers for USD $1k.
What about Soekris hardware? I don't have any personal experience with it, but it looks very appealing to build load balancers/routers out of, and quite inexpensive.
Before you choose which hardware platform to use, you should take a look at the software platform and see what other people are using. There are dozens of Linux router distros like OpenWRT out there. http://leaf.sourceforge.net/ Linux Embedded gateway/router/firewall http://www.linuxdevices.com/articles/AT6003080606.html Building a low cost router appliance Linux Devices is a good site to find information about embedded hardware platforms that support Linux. There are a lot of possibilities ranging from fanless x86 systems built around a Via EPIA motherboard to traditional embedded platforms based around ARM or MIPS processors. And just about anything that runs Linux will also run BSD if that is what you want. --Michael Dillon