I'm not aware of an official route to the Keys. The ACA maps go as far as Fort Myers on the Gulf coast. Also, it is not possible to reach Key West, by bike, from the gulf side. You can ride south only as far as Everglades City/Chokoloskee. Sooner or later, you would have to cross the state to reach the Keys.
The southernmost crossing available to cyclists (or motorists) is US 41...the Tamiami Trail. This is a high speed, two lane road running from the Naples area across to Miami. If you were to cross the Trail, then Hwy. 997 (Krome Ave.) would be found running north/south along the outer fringes of the Miami sprawl. A right turn here will lead to Homestead.
The Homestead/Florida City area is the hub for all points south. From there, you have two choices to reach the Keys. The main drag (US 1) and the alternate route over the Card Sound bridge. Both of these routes can be heavily used at times. Though much of it lacks a shoulder, local cyclists seem to prefer Card Sound Road.
From Orlando, you could instead work your way to Lake Okeechobee, then grab US 27 south. This will take you south, skirting past Palm Beach and Broward counties. Before reaching Miami, you can branch off and take 997 to Homestead. Lake Okeechobee can be a very nice diversion. The entire dike surrounding the lake has a recreational path for cyclists and hikers. Much of it is paved as well.
Here are a couple of links to articles on South Florida touring...
http://www.stubna.com/cycling_florida/http://www.floridabicycle.org/messenger/msgronl0103/winter038.htm