I did mine in 2001 through my solicitor. It took only 4 months until I was issued the Oath of Allegiance, but there was then a final wait of many weeks for the certificate (because the certificate printer had broken down, if you can believe that).
Anyway: When I applied in 2003 for a new US passport, I expected to hear what they told Sam, and I had written up a letter stating my intent to retain US citizenship. When I went to the (Munich) consulate for making the application, I had filled in the application to show that I had taken another citizenship since last applying etc. etc. The consular lady just said, no problem, we don't need to see your British passport, that will be $85, please.
That was it.