It's not that hard. Someone can be monitoring, mastering, and tracking throughout the show. With just 2 or 3 High-speed, high-capacity CD-R duplicators like this:
http://www.stordigital.com/roboticdvdcdcopy1000.html
you can pump out 300-400 discs per hour. Those duplicators only cost about $2000-3000 each. The packaging and CD-R's can be pre-printed and pre-silkscreened.
According to their website, Live Here Now has been doing this since Dec. 2004. I think InstantLive was also doing it at least since 2004, because they did all the PWEI Reformation gigs in Jan. 2005, including a DVD release at one of the shows.