There are many miles of nice bike path along the keys, BUT they are far from finished construction and you'll be forced to ride on traffic lanes for much of it.
At least that was my experience two years ago. The ride is well worth it regardless.

http://www.flickr.com/photos/2797683...n/photostream/
This pic (and the one before and after) are from here: http://goo.gl/maps/iXDyR