Thanks guys.
And an update on the ceremony: in my area the next group ceremony is at the end of October, so instead I'm paying £75 for a private ceremony. The woman on the phone said I could go in same-day, but that wasn't enough notice for my in-laws, so the next slot was for Tuesday. If we had more time I would've either waited for the group ceremony or switched to a different office, but I've decided that at the moment I accept British citizenship I'm going to relinquish my US citizenship (don't worry, you can't do it accidentally, it involves filling out forms and a trip to the US embassy, etc.) so in our situation it was better to pay the £75 to be expedited. I haven't seen any recent posts in this thread about private ceremonies, so just wanted to make people aware of them in case it's useful.
NCS or Postal: NCS Worcester
Date of submission: 06 June 2012
Basis: Three years in UK with British spouse + ILR
Date payment taken: about 1 week later
Date of acknowledgement letter: 13 June 2012
Date of receiving acknowledgement letter: 19 June 2012
Date of decision: 09 September 2012
Date of receiving letter: 18 September 2012
Date of ceremony: 25 September 2012