ISPs don't have all the power IMO, b/c they don't work together . If comcast blocked me from doing something I wanted, I'd just switch to another ISP. I'm surprised that Comcast went out on a limb like this . . . they're probably affiliated with some big content provider, and got leaned on.
In most area's you have two choices for high speed internet access, those two forms of connecting to the internet are typically controlled by two companies. For example, Verizon and Comcast or Verizon and Cox.
If both Verizon and Comcast choose to block certain ports in order to stop P2P file sharing then there's nothing any of us users can do about it.
As I said, as it stands right now, they hold all the leverage.
Of course you can always go dial-up and spend 4 days downloading a 400 MB file.