Image by Google Maps
It looks like Google has just flipped the switch on offering directions by bike on Google Maps.
I did some quick mapping tests for Charleston and generally it does a pretty good job. The service prefers secondary roads where possible and picks up on bike/pedestrian paths like the West Ashley Greenway.
However, as with all such services, it's not perfect. Take for example that it refuses to recommend a route on East Bay Street leading up to the bridge where the city has a sidewalk meant for bikes. Instead Google suggests you bike down the parallel Drake Street -- an alternative that could be rather harrowing for tourists. That's among other odd routes.
Still, in the end it seems to be a worthy asset when looking for routes to places you've never biked before -- and as with most technology: it'll probably get loads better.