I think we have to a little bit separate motivations and outcomes. Are we taxing to change people's behavior, to reduce fuel consumption, to improve the environment, or just to make money? I find that most taxation is for the purpose of making money, plain and simple.
In the case of the proposal to tax mileage, I think it's all about money. It may reduce OR increase fuel consumption and emissions, because if people drive gas guzzlers, that could outweigh decreased mileage. It certainly could kill one of the incentives for fuel efficient vehicles, and chill alternative energy research and production, which is sorely needed. Of course, some people will buy hybrids not for fuel savings, but for environmental concerns, simply to decrease consumption of fossil fuel, but I do feel, if the environment is a priority, then it is foolish to create decrease the incentives for fuel efficient vehicles.
I also believe that it's unfair to people who must travel longer distances to work, particularly in a climate where good jobs are not readily available anymore. Most people would LOVE to work near their home. Hell, they'd love to work IN their home, but it's not feasible for most of us. When I had my own business, I could walk to my office. The business faltered, and I was forced to take a job...in fact TWO jobs, in order to earn a living. Now I have to travel up to 30 miles per day. I can't just move to follow my job. And if so, which job? And should I uproot my kids every time I lose my job? Or should I consider a less optimal school district? And what if moving closer to MY job is further from my partner's job? It's just not that simple.