While I have not suffered any dents yet, I have investigated the same thought myself.
Having the unibody replaced is probably the hardest thing to do, and it would not make any sense financially.
Unless it was to be performed by Apple, or an authorized dealer, it would void the warranty as well.
The interesting catch with the unibody is that even if you want to replace a damaged keyboard or key, Apple considers it one piece, and they would replace the entire unibody, which would pose a significant cost.
You should review the following link for more interesting information.
http://www.ifixit.com/Guide/First-Look/Mac/MacBook-Pro-Unibody
Should I learn anything else, I will certainly write back here.