I am a Comcast user in the Midwest, and I performed the recommended test specified here:

http://www.limewire.com/index.jsp/refund

The test failed, thereby proving that Gnutella is being blocked. One thing that was curious, I did manage to attain an established 6346 connection with an ultrapeer for about a minute, but then it disappeared, and I haven't gotten another connection. I'm assuming Comcast is blocking Gnutella nationwide.