If after you update to 4.2 your iPod app seems to be devoid of media...
After updating to 4.2, my iPod app was empty, but in iTunes all of my music, podcasts, and video appeared to still be on my iPhone. I disconnected my iPhone and restarted it. The iPod app was still empty. I reconnected it to my Mac and iTunes still showed the media. Then out of curiosity I selected a song on my iPhone in iTunes and hit play. The music started playing on my Mac and my iPhone lit up with the 'syncing' message for half a second. I reopened the iPod app on my iPhone and for a second or two it displayed the 'updating library' message. All of my media was there.
Just FYI.
32GB Black iPhone 4+3.06GHz 17-inch MacBook Pro 8GB - 500GB@7200 - Antiglare -, Mac OS X (10.6), - I want .mac back as an email option for MobileMe.