iPad and iPod touch suddenly require activation
This problem happened as the iOS 6 beta 4 expired today, so all devices that were running that version of iOS will need to be updated to the non-beta version. As iTunes and iOS don't see a difference between the two (as they're both called simply "6.0"), this update needs to be done manually, with the following steps:


Download the appropriate version of iOS 6.0 from the iOS Dev Center.
Connect the device to iTunes.
Option-click (Shift-click on Windows) the "Check for Updates" button on the Summary tab of the device in iTunes.
Select the .ipsw file in Finder and choose Open.


Your device will then install the update and everything should be fine.