Here is a great tip I found on the Apple discussion boards courtesy of Neil Barman.
You can use Numbers to export your address book entries to a comma separated values (csv) file.
Here’s how you do it:
Open Address book, then select the entries you wish to export. Drag them into numbers, on a blank area of the canvas, and a table will automatically be created. By default, the following fields are displayed: Last Name, First Name, Phone and Email. The other address book fields are present, but hidden. To have all fields display, choose the Unhide All Columns from the Table menu.
You can then select Export from the File menu, and choose a csv format that will be suitable for import into another application.