I run Ubuntu 9.04 (Jaunty) and KeePass Portable 2.0 works for me. It also appears to work for others from the later responses on this thread: [SOLVED] Mono & KeePass 2. People who initially complained about crashes have reported it to be working for them on the final Jaunty release.
As for installation, get the latest Portable Keepass Professional edition and follow the project instructions on running KeePass under Mono. If you have the mono components required, you should be able to unpack the zip and just run the KeePass executable.
The forum post linked to above mentions that you need the libmono-winforms2.0-cil and libmono-winforms1.0-cil packages in addition to the core mono package. However, I only have libmono-winforms2.0-cil on my system and Keepass 2.0 works fine on it.