The push notifications aren’t really a big deal for me, as I get email alerts for the notifications which I want from Facebook. But the contact-syncing feature is extremely useful, and if you sync your iPhone contacts to another source (in my case Google Apps) you’ll have consistent information there too.
And Push Notifications aren’t the only new feature of 3.1. There is also a new way to sync your Facebook friends with your iPhone’s address book. This includes replacing contact photos. Pretty nifty.