I tried biking to work for a while when I lived in Medford, but riding on Mass Ave was a sufficiently awful experience that I stopped doing it. All the biking lit I've read emphasizes finding alternate routes that have less traffic.

And yet, there were always idiots biking up and down Mass Ave with no helmets and their iPods on, and they didn't get killed...

There are some nice bike routes down by the river in Alexandria. I'm pretty sure you could bike into Mount Vernon if you wanted to, as well as into the city.