Is it possible to detect a visitor's ISP, could websites educate Comcast users?

As I understand it, there are limitations, it will not be 100% accurate ie: a user could be using a proxy, or VPN, etc.

Still, if Comcast is going to throttle specific websites, or charge specific websites for faster connections, can't we use the knowledge that a website visitor is a Comcast subscriber to our advantage?

More than that, could Reddit use a protest day to demonstrate what may happen to Comcast customers?

For example, instead of a blackout day, Reddit loads slow for Comcast subscribers and only Comcast subscribers.

For programmers, here's a possible solution using PHP: http://stackoverflow.com/questions/855967/how-do-i-retrieve-the-visitors-isp-through-php