According to http://noc.sixxs.net/pops/, there are no SixXS POPs outside of Europe. However, there are a couple of US IPv6 tunnelbrokers as well, including Hurricane Electric.

I'm a little late in responding, but I'm in Michigan, and signed up for a tunnel in Ireland...the latency was high (~300ms iirc) but I was approved anyhow. I can heartily recommend http://www.he.net for use in the US. I have a tunnel through them and the speeds/latency is quite decent. I'm very anxiously awaiting the time when we will have native IPv6 access here in the US...probably going to be a long wait. :( Funny how I didn't get interested in IPv6 until I read a thread on the Debian mailing list (which was wrong) about Comcast giving IPv6 addresses. I spent quite a few hours trying to get it to work. I didn't realize that was a local only IP. :P Thanks to http://lists.debian.org/debian-user/2004/02/msg02190.html, I'm now on the 6Bone though. :) Again, in the US try, Hurricane Electric (http://www.he.net). Good luck.