It wasn't planned or announced. You had to have been there. He tweeted about 100 times with fun insider observations.
I found out about it early this morning from Bob Lefsetz.
Lucky fans who were 1) watching the movie on HBO, 2) simultaneously had Twitter going, and 3) were following Stern were in the real-time loop.
Some had their questions answered live.
I loved Private Parts (both the book and the movie) when they first came out. I'm going to re-watch the movie while checking an archive of yesterday's Twitter stream. Download a PDF of the stream here.
Great stuff Howard.