You should probably go to forums.comcast.net and post some traceroutes. There's no latency penalty from the SF bay area, and Alabama seems like the kind of place that's easily overlooked.
Sorry, but this is untrue for me. I have Comcast, and if I enable IPv6 from my home in Santa Clara county, I also start seeing large increases in latency and significantly slower download speeds.
I haven't bothered posting on the Comcast forums. I simply disable IPv6 and get on with my life. Maybe I'll give it a try in another few months.
The "no latency or speed penalty over IPv6" thing is true for me in San Francisco for major sites (Google, HE.net, Facebook, Wikipedia).
Are you sure that you're not running over a Teredo tunnel? (Is the IPv6 address you're handed in the 2601::/28 range [native IPv6 for Comcast], or is it in the 2001:0::/32 range [Teredo tunnel]?)
Does some of your networking hardware handle IPv6 poorly? What happens when you plug in a IPv6-enabled computer directly into your cable modem?
