You'll find that you can say you're launching even if the functionality has already launched a few days before hand.
This has been the case for some iterative launches for me, so maybe it's different for a site launch.
We launched Tipjoy at a publishing conference. It wasn't a good idea, as TechCrunch posted a story a bit early while we were on a flight. I think my exact thought at the time was: "fuck!"
If I were to do it again, I would have an open-to-anyone site for a few days before "launching" at the conference. I fixed more bugs in the first 24 hours of launch than the previous week of development. (and keep in mind this dev time was mid YC push)
And when you were coordinating the conference push, was it actually integrated or announced at the conference? Or just alongside the buzz with some other form of PR on the same day?