[Joel]

Starting with OmniGraffle 4.2 (to include OmniGraffle Professional), we named the application "OmniGraffle 4" (or 5) and then localized in the English localization to only display in the Finder "OmniGraffle" or "OmniGraffle Professional" in order to expressly not overwrite an older version of the application.

Basically even though we warn users that a major upgrade (i.e., version 4 to 5) will require an upgrade fee, too many people wound up upgrading and stuck without a valid license key -- Hence the desire to download and install a newer version that wouldn't overwrite the older version of the application, leaving the licensed version intact.