Ok, so this has driven me mad since yesterday too (again ipad2) - and i had tried the suggestions outlined above and similar ones on other related threads with no success.
However the link above in jwmesser post has finally worked for me! I had tried this before (as another post somewhere had the same link) and on the first go i opted to Update - but that just took me through the whole install cycle again to end up with the same problem and unable to activate the iPad. so on 2nd attempt i instead opted to Restore. After i selected the backup file to restore from, the following message said it would Restore and Update - so i went with it and although eventually success!! Yes a pain that I'm now waiting for all the apps, content etc to synch, but at least i have a functioning iPad again.
More painful that it needed to be but at least the good folk on here are able to help each other out where Apple seem to struggle!
I know solutions don't always seem to work for everyone, but give the above a shot and hopefully this will work for you too.