EDIT: As of Ubuntu 11.10 (Oneiric Ocelot) this is now much easier as KeePass 2 has finally made it into the repositories

apt-get install keepass2

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If you need to read KeepPass 2 data files (.kdbx) on Ubuntu (as well as from other platforms such as Windows or Android) you need to run the Portable Version under Mono (.NET Runtime). Make sure you download the Portable Version 2.x from http://keepass.info/download.html

The default Mono Distribution on Ubuntu (V11 Natty) is missing a few dependencies required for KeePassX