After upgrading my desktop to Karmic Koala I started having some problem with my Notes 8.5.1 installation with window not displaying contents. Doing a brief Google search I found this thread about a change made in libgtk that breaks Lotus Notes.
The solution is in comment #13: just follow the simple instructions and Lotus Notes will be working again.
Update - Nov, 4th 2009
As pointed out by jklocke, a fresh Lotus Notes installation on Ubuntu 9.10 doesn't work: Notes won't start at all. The solution he found is very simple and quick: Notes cannot find the packages
To resolve the problem run this command to install missing libraries:
sudo apt-get install libgnomeprint2.2-0 libgnomeprintui2.2-0
Update - Mar, 30th 2010
Some commenters reported that they couldn't see checkboxes status in preferences. The solution found by ElToro and confirmed by Perin is to switch the theme to New Wave.