This isn't a Mail or Address Book question, so this isn't the best forum in which to address this. But this is what Nikon's support site says about Nikon Scan and Leopard:
"SUPERCOOLSCAN 9000 ED, SUPERCOOLSCAN 8000 ED, SUPERCOOLSCAN 5000 ED, SUPERCOOLSCAN 4000 ED, COOLSCAN V ED, COOLSCAN IV ED digital film scanners.
Our initial testing of Nikon Scan version 4.0.2 and all scanners listed above showed no problems. Preview, Edit, Scan, and Save operations were successful with both USB and Firewire devices. Installing the 4.0.0 full version, then the 4.0.2 update took a little longer than usual to complete, but there were no problems. Since there is an issue with the installer, Nikon Scan is not considered to be compatible with Mac OSX version 10.5. "
Nikon is notoriously slow to provide updates to their software, so if you encounter any problems, a fix in the new future is highly unlikely. In such a case, you might want to consider SilverFast or another scanning solution rather than use Nikon's software.
Regards.