Google maps let's you plan out your route and get directions by car - why not bicycle?
Here's a petition you can sign asking them to add bicycle lanes to their maps! I was number 33,704. Will you be number 33,7xx?
A mashup of this is already available for the following cities:
New York City: http://www.nycbikemaps.com/
Portland, Oregon: http://bycycle.org/
Milwaukee, Wisconsin: http://bycycle.org/
via The Backcountry blog.