I had this issue, and was able to re-activate without losing anything.
- do not backup this busted version
- de-activate the "automatically sync when this device is connected"
do a restore as a new phone to confirm your hardware's OK:
- download the image you'll need (for example, http://www.iclarified.com/entry/index.php?enid=750 and scroll to the bottom, in the "6.0.0" items)
- download it, and right-click or option-click on "Restore", select that ipsw file, and install a new phone (not a backup of your old phone)
- this may take 40 minutes -- let it do its thing
- you should end up with a very basic set of icons, so past SMS, no addressbook
- confirm you have an activated phone by sending/receiving SMSs and calling or answering calls -- your hardware is OK, let's go restore a backup
restore the existing backup of your phone:
- right-click or option-click on "Restore", select your ipsw file again, and choose to restore from a previous backup of your pre-busted phone
- let it do its thing. 40-50 minutes, longer than above because it's restoring your previous apps, SMSs, addressbook, etc
- when complete, you should have a functioning phone with all your addressbook and SMSs, but content (photos, music, videos) still need to be sync'd.
Step 3.2 requires that it'll let you restore -- if you keep getting the "check for a more recent version", try also DFU mode, and when it asks you to restore a software version, choose the IPSW file you downloaded in step 3a.
DFU can be done by:
1. hold the power/lock button on top while holding the Home button on the front
2. do for 10 seconds
3. release the top/power/lock button while still holding the Home button
4. do this for 10 seconds
5. release the home button, so no buttons pressed
6. the device will appear as "iPhone" in the devices (your old device may still appear for a few minutes) ready for a restore