Update: We deleted installer.app on ours and kicked off the update, things are taking a while but no obvious problems yet. Stay tuned.
Update 2: Success! The phone's restarting now.
Update 3: WiFi Music Store, check. New calculator icon to match the iPod touch, check. Phone had to be reactivated.
Update 4: We're hearing numerous reports of "Incorrect SIM" errors on unlocked iPhones after the update -- user beware!