UPDATE: IBM has posted instructions and a tweak for your Lotus Notes client on OS X 10.8. I recommend following the instructions posted by IBM, rather than my tip below.
This tip comes courtesy of an excellent suggestion by @stuartmcintyre… After installing the OS X Mountain Lion GM seed, Lotus Notes gets moved into a Lotus Notes folder. On my machine, double-clicking on Notes.app within the folder causes Notes to start but when I double-click on an email or calendar appointment, Notes would crash. A solution is to move the Apple Notes.app file into a folder and move Lotus Notes back to being the Notes.app application (into the same location as it was before the upgrade to Mountain Lion). Finder won’t allow you to move or rename Apple’s Notes.app so terminal is needed: Open Terminal in Mac OS X then type the following commands:
sudo mkdir /Applications/Apple
sudo mv /Applications/Notes.app /Applications/Apple/Notes.app
sudo mv /Applications/Lotus Notes/Notes.app /Application/Notes.app This has worked well for me, but as always, make sure your backups are up to date before doing this! Thanks Stuart!