Want to add support for your Nokia N91 to iSync? Here's how. In the Finder, control-click on the iSync application and choose Show Package Contents. Then navigate into Contents -> Plugins -> ApplePhoneConduit.syncdevice -> Contents -> Plugins -> PhoneModelsSync.phoneplugin -> Contents -> Resources. In that folder, modify MetaClasses.plist (make a backup first!) using the below code.
Here's what you need to add to the file:
<key>com.nokia.N91</key>
<dict>
<key>Identification</key>
<dict>
<key>com.apple.cgmi+cgmm</key>
<array>
<string>Nokia+Nokia N91</string>
<string>Nokia+Nokia N91-1</string>
</array>
<key>com.apple.gmi+gmm</key>
<array>
<string>Nokia+Nokia N91</string>
<string>Nokia+Nokia N91-1</string>
</array>
</dict>
<key>InheritsFrom</key>
<array>
<string>family.com.nokia.serie60v2.3</string>
</array>
<key>Services</key>
<array>
<dict>
<key>ServiceName</key>
<string>com.apple.model</string>
<key>ServiceProperties</key>
<dict>
<key>ModelIcon</key>
<string>NOKN91.tiff</string>
<key>ModelName</key>
<string>N91</string>
</dict>
</dict>
</array>
</dict>
I've also found an icon to use for the phone.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060509170202873