Downgrading iPad from 4.2 to 3.2.2 on OS X

To restore your iPad from a newer version (beta, perhaps?) of iOS to a previous version of the OS (such as 3.2.2, the latest release), do the following:

1. Connect your iPad to your computer
2. Open iTunes
3. On the left side, select your iPad under Devices
4. On the Summary view for your iPad, option-click Restore
5. Browse to ~/Library/iTunes/iPad Software Updates and select the .ipsw file (it should say 3.2.2 somewhere in the filename)

Thanks to this post for showing me where to find the .ipsw file.

Update: If you've lost the old .ipsw file for 3.2.2, you can download it from Apple at this link.