To add support for the Sony Ericsson S500i in Address Book, do the following. In the Finder, control-click on Address Book and choose Show Package Contents from the pop-up menu. Navigate into Contents » Resources, then control-click on Telephony.bundle and again choose Show Package Contents from the pop-up menu. In the new window that opens, navigate into Contents » Resources, and then edit ABDeviceCommandSets.plist in your favorite pure text editor (perhaps make a backup first, just in case). Search for K700, and when you find it (it will look like <string>K700<string>), add this new entry below it:
Save the changes and quit the editor, and you'll now be able to dial and send SMS directly from Address Book. It will also enable you to pick up the phone from Address Book.