Before I start, I'd like to mention that this is mostly hypothetical and doesn't involve my current roommate. Some of the thoughts were spurred by an amiable conversation we had.
The tragedy of the commons is a frequent topic of conversation when environmental justice is discussed. Succinctly, the main issue is that there will be difficulties caused when many people try to share a common yet finite resource. Any benefit of drawing from this resource is realized in full by one individual. The drawbacks of drawing from this resource are mitigated, as any negative impacts are spread among all users of the resource. We use this metaphor to talk about any sort of environmental damage or overuse of resources that comes from companies and individuals not fully bearing the brunt of their decisions.
In roommate terms, the tragedy of the commons can be appropriately applied to utilities in a common living situation. Typically, roommates might split rent based on square footage but split utilities straight down the middle, under the assumption that each person is using more or less the same amount of electricity and water. However, in my experience of living with roommates, this is sometimes not the case, and I actually think that both "utilities included in rent" and this tragedy of the commons are two reasons why people feel no incentive to conserve utilities.
While utilities included in rent is obvious (you never see the electric bill, so why worry about how much you're using?), the tragedy of the commons seems less apparent to many people. But the tragedy comes when people feel uncomfortable discussing how the cost of utilities should be allocated. In fact, I even asked this question in an online forum, and the general consensus was "just split down the middle; it's not worth it to split hairs about utility usage." And I agree that paying for energy use based on exact cents used doesn't quite make sense, but it does chafe if one roommate is exceedingly eco-conscious and uses little electricity while another roommate doesn't worry at all about energy consumption.
To relate this to the tragedy of the commons, you can think of the gains and drawbacks this way. When one person conserves, that person is taking all the drawbacks onto themself. For example, you might love to have a cool room and run AC all summer, but you realize that it's going to be expensive and therefore don't do so. However, the positive -- in terms of a lower electric bill -- is split between yourself and the roommate, so you don't realize the full benefit of your decision. In the converse, a roommate that uses a lot of electric in any way realizes all the gain from utilizing the electric, but that roommate only pays half of the increase in electric due to his or her own choice. My main thought is that this reduced impact of high electric reduces the incentive for any one individual to act in a conservation-minded direction.
I haven't decided where my personal tipping point is. I know one thing: I do refuse to pay for someone else's air conditioning use, since I would never use it myself. We've got wall AC units in each room here, and they cost about $2 a day, so the plan for the summer is to be honest and pay extra if someone used the AC. When I was in New Zealand and visiting my brother, I noticed the same thing about dryer use. Their dryer had a sign on it, and each roommate marked down when they used the dryer and kicked in $2 to the electric pot. Whatever was left over after dryer differentials was then divided amongst the roommates, and it gave an incentive to hang dry clothes rather than pay the extra cost.
I really think this sort of differential could and should be applied to US living situations. It's a tricky issue, as a lot of people feel entitled to live as they wish. And I definitely know how hard it is to talk about a living situation problem with a roommate, especially if you weren't friends with the roommate beforehand. But it seems to be a necessary step toward making the cost of individual actions clear to all the players.
The bottom line, for me, is that working toward a greener nation is going to require a lot of sacrifice from the citizenry. And in order to encourage individuals to make those sacrifices, each and every individual needs to realize the full cost of their own personal decisions.