By "blank firmware state" I mean it is if you had purchased your iPad, but the firmware was upgraded to 4.2.1. But you DON'T restore a previous iPad backup.
In iTunes you do the following:
1. Connect your iPad
2. Click the Restore button in the iPad settings window (I don't have iTunes in front of me, so I don't know exactly what it is called).
3. Let iTunes wipe the iPad clean.
4. The iPad will eventually have the same "plug me into iTunes" picture that you had when you bought it. When that happens, unplug your iPad, then plug it back in.
5. iTunes will recognize the iPad, and will tell you that you can restore a previous iPad, or you can add the iPad as a new device. You add it as a new device.
This gives you what is essentially a clean installation of 4.2.1., because in this process, iTunes has installed the latest version of the firmware, NOT the version of the firmware that was on your device when you bought it.
Now you will pick what music/apps/movies/podcasts/etc. to add to your iPad, and it will sync.
When that process is complete, you should have all your apps and music, but 3G will work.
In this process, however, you basically wipe the underlying databases clean for all the apps. That means that you lose all application data, settings, content, high scores, etc. But--at least in my case--3G works again, so I decided it was worth it.
