There is a issue with KeePass in Ubuntu 10.10. As I had to fix this on several computers, I am posting the solution here. It was somewhat inconvenient to track down.

Run this command in the terminal:

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sudo apt-get install libmono-winforms2.0-cil libmono-system-runtime2.0-cil

Earlier you only needed to install the winforms package to run KeePass, but with Ubuntu 10.10 it seems you need the system-runtime as well.