While a shift from gas to coal does reduce dependency on foreign oil, they also are like "trading one greenhouse gas emitter for another," said Mark Gaffigan, a co-author of the report in an interview with CNS News (the site bills itself as an "alternative to the liberal media"). The GAO report suggests that new nuclear and carbon sequestration technologies could help reduce the amount of greenhouse gases that are emitted even if plug-in cars become commonplace. It also says that nighttime charging from coal plants will likely reduce the amount of smog formed by powering plug-in vehicles. The GAO says that, "Besides offering environmental benefits, reduced oil consumption from plug-ins could help to limit U.S. vulnerability to supply reductions and subsequent oil price shocks" (page 13).
Overall, the tone of the report is much less alarming (or surprising) than the CNS News report makes it appear. But not everything in the report is accurate. On page 19, for example, they still list the Phoenix all-electric truck and SUV as making an appearance in 2009. That's maybe possible, but very unlikely. You can download a PDF of the GAO's report here.