I stumbled upon this blog post which claims that installing foam gaskets and plugging your electrical outlets with plugs will reduce your heating costs by reducing air leakage. Is this true?
Probably not. Air infiltration in houses is measured by a "blower test" in which a fan is installed in a door, and used to draw air out of the house. Here is a discussion and video of air inleakage through a gasketed switchplate during a blower test in a home. The artcle is on EnergyVanguard.com, which is a site and organization focused on energy efficiency in homes, with a lot of attention focused on air infiltration.
The author explains that air infiltration through electric boxes is a real problem, but the gasket doesn't help much because it's really not sealing the places where leakage occurs:
Several years ago, I put gaskets in all the switches and receptacles in a house. The before-and-after blower door tests were pretty much identical (within the uncertainty of the equipment). So why don't gaskets help much?
The main problem is that gaskets seal the parts that are already sealed. Air doesn't leak in through the plastic cover itself, yet that's where most of the foam is. They do help reduce leakage a bit for covers that don't fit snugly against the wall, but they do next to nothing about the holes in the switches and receptacles.
The article goes on to explain effective ways to reduce air infiltration in electric boxes.