KrisJ:
If you are going only for the purpose of eating at the Rainforest Cafe, I would suggest the same scenerio Dopeysgal suggested...
Rather than walk clear back to the front of DL to exit, cross the entry way, and then walk through downtown Disney, I would plan your schedule to be in the Tommorowland area before you leave for the Rainforest, then just catch the monorail there and get off in Downtown Disney. From there it's really close.
If you're walking, once you get into Downtown Disney, it's almost all the way to the end of the "road" to the RFC, and if you haven't been there before, you may be tempted to stop in a few shops or look around (which could be an option too). After dinner hop back on the monorail there and continue on from where you left off in DL earlier!