It's a tough situation :). On the one hand, it isn't as good as it used to be (as you point out with the water company analogy). On the other hand, that also points to the fact that people can just be wasteful of what is a communal resource - or they can just use more because they're ridiculously thirsty people, either way, less for others/more required.
Hopefully, Comcast will keep the cap high and offer rates scaling up linearly (or preferable logarithmically).
I guess I'm a bit luckier than most people since I have the option of FiOS, Comcast or RCN and both Verizon and RCN offer service at 20Mbps (with Comcast being the slow-poke at 8Mbps). Most are limited to cable or DSL. So, I don't have the same concern on a personal level that Comcast will decide in 6 months to move the cap down to 150GB or 100GB or 75GB. I just have other options.
If Comcast tries to pull that, I hope the government steps in since they do have monopolies in most areas they serve (because who would use DSL). Maybe a nice formula specifying norms and standard deviations could be created that would automatically set and raise levels as people's bandwidth consumption increased? That sounds like a good plan? This way providers have to keep up with the times (so they can't just start offering crap service hiding behind caps), but it still deals with the issue that there are some people who just use, use, use way outside the norm.