Good question.
There's a web site discussing this here.
While cost savings doesn't seem that dramatic (although it looks pretty damn good with gas at over $3.00 a gallon), the article points out other advantages to electric vehicles, particularly in the use of domestic energy supplies. As the share of alternative energy sources and nuclear goes up, this looks even better.
And remember, electricity produced at large coal plants at least has the option of things like sequestering CO2. Cars that burn gasoline don't.