I’ve been attending VON this week as well. (Voice on the Network) Mark spoke at a lot of the conference sessions that I went to.
Ravi Sakaria of Voicepulse spoke there as well on a panel with Mark and mentioned that he uses Asterisk almost exclusively.
He also mentioned that they did some fairly extensive changes to the code to accomodate (among other things) larger SIP registration limits per server.
I was somewhat dismayed to hear that he has no plans to contribute the improvements back to the Asterisk project. (He cited the standard “we did it, it gives us a competitive edge, and we don’t want the other guy to have it” excuse)
I know that large providers are making use of the software. Almost every provider who spoke there so far has mentioned it. However, I wouldn’t expect great improvements to the code because of it. They’re going to keep their trade secrets to themselves.