I spent some time this evening making sure that my Windows 7 laptop wasn't directly accessing 6to4. I wanted to make sure that the Win7 computer was properly routing traffic to the router - and the router then pushing data through the tunnel.
First thing was to disable the IPv6 traffic through tunnel interfaces. The fix-it programs on the MS website are quite handy for this job (See KB 929852). After rebooting I used regedit to view the local settings for the PC (as described on the KB) just to verify all settings.
Because IPv6 was previously enabled using the full-tilt option, everything continues to work. Using a tracert, nslookup, and ping, I verified that traffic is indeed flowing through the gateway address (the router).
The reason I disabled the local 6to4 was because the Win7 computer could have tunneled through the IPv4 address of the router. So even if the new modem allows native IPv6 the Win7 PC may have continued to seek out the network using the 6to4 adapter. These kinds of changes should be encapsulated at the router thus enabling access for all devices.
In other news, I purchased the Motorola SB6121 SURFboard modem. That should be hooked up and ready to go by the end of this week. I hope to see if native IPv6 is available in my area this weekend.
Comcast has also updated their blog site indicating support for World IPv6 Day 2012 as well as announcing a pilot of IPv6 in select California markets. See http://www.comcast6.net/.