Being a mildly suspicious type, I turned to the environmental publication Grist to find out whether this was a scam. According to what I found, cleaner power consists of something called a Renewable Energy Certificate (REC).
Since this is a new business, Grist suggests that we check any offer of "clean energy" against a third party verification site, Green-E. And CleanPowerSF is certified; apparently our offset will be 100% wind power.
CleanPowerSF is a joint state/city/utility program, part of California's ambitious sustainability efforts. I like that. Individual efforts aren't going to solve fossil fuel pollution; we're going to have to make society-wide efforts to have a chance. And the simpler we can make it for people to help out, the better. When fully implemented, San Franciscans will have to affirmatively opt out to avoid clean power! RECs may be available from private vendors in other places. Check the Green-E site.
Here's a simple video which does a pretty good job of explaining about RECs.