Push Notifications
When 3.0 came out this summer, there were some vocal complaints that the Facebook iPhone app didn't do push notifications; it seems like basic functionality for any post-iPhone OS 3.0 application. Now when you start the app up after updating, you'll be prompted asking if it's okay to do notifications. Once you've said yes, it follows the standard iPhone notifications rules. You can customize what you'll receive in settings.
Address Book Syncing
Right after you allow or deny notifications at the beginning, you'll be asked if you want to sync with your address book. It will sync personal data, and change your contacts' listed homepages to their Facebook profile URLs if they didn't list anything themselves. You can choose whether or not to sync profile pictures, but that's the only customization option you have. Still, it's a very welcome feature.
A Minor Quibble