Ahh... Yes, I neglected to mention the firey moat I put at the bottom.
On a slightly more serious note, I certainly don't think it's impossible for a Chameleon to climb down from the canopy. However, the lack of cover (i.e. leaves) and secure footholds (i.e. branches) on the bottom ~1/2 of the tree really seems to discourage my Chameleon from even trying.
It's probably situational though... My Chameleon is still pretty young, which I'm guessing means he's still somewhat limited in his climbing abilities. Plus, he only free ranges while I'm cleaning and/or rearranging his enclosure. So, there isn't really much time for exploration.
That said, I'll continue to make sure he's supervised while free ranging.