cmd-' will open the notes field and start editing, or close the notes field if you are already editing. If you look at the archive of release notes from way back around the start of the 1.1 sneaky peeks, you'll find this nugget:
The Enter key now saves edits rather than editing notes. You can still edit notes using Command-', but if you prefer using the Enter key you can set the OOShouldEditNotesOnEnterKey preference to true using this command:
defaults write com.omnigroup.OmniFocus OOShouldEditNotesOnEnterKey -bool true
If you'd like to start weaning yourself away from using the Escape key to save edits, try this:
defaults write com.omnigroup.OmniFocus OOShouldToggleEditingWithEscapeKey -bool false