I came across FacebookSync yesterday on TUAW, the application allows you to synchronise your Mac's Address Book with all your contacts in Facebook. For the majority of my contacts it added pictures, email addresses and birthdays to my current contacts and for those that weren't in my Address Book it added new contacts.
Most of my contacts are on Facebook and the program did a nice job of matching contacts up. For those that it couldn't match I had to select contacts then merge so that there were not duplicate entries.
Once synchronised I could use iSync to copy all the updated contacts to my phone. (iSync supports most Nokia, Motorola , Samsung and Sony Ericsson phones, for Windows Mobile based ones try Missing Sync).
Overall its a nice utility that makes good use of Facebook.