when I "lived" in Canada for over a year after the UIGEA was passed I had this problem several times with betfair, but was fine logging into ipoker. I was on a VPN server that would be impossible to tell apart from a normal Canadian computer. The thing is, ISP's often route through major hubs as far off as Atlanta. When you have a site like betfair who uses poorly written scripts to sniff out people playing from the US, you get false positives like this.

Fortunately for me, my vpn was in the business of making it look like I was from Canada, so when I let them know that their routing was signaling a false positive, they click a few buttons or whatever they do and voila, I was back online. For you.. I really don't know what to say. The issue is most certainly with your ISP, but even if they understood the issue I'm not sure they could reroute just your connection as easily as this VPN service could.

p.s. Does betfair's software still suck a fat one? Having to click a button to be able to type in a bet was the most absurd thing I've ever seen on a poker site.