I am happy to report that the 72-hour transit stamp in Beijing is indeed and truly present. It started on January 1st, 2013. We came into Beijing from Bangkok, arriving around 6:30 a.m. on January 1st, and thus were among the first ones to utilize this. A film or TV crew had just been there to record the very first travelers who came through.
The process could hardly be easier. There is a clearly marked booth, in Chinese and in English, saying "72 Hour Transit" or similar. The sign is in bright orange LED lights, and is distinctive from the signs over the other booths. At the current time, it is located on the left side of the big room that the OP refers to, where the immigration and visas booths / lines are.
At a minimum, you need a passport and your through-ticket showing when your flight leaves China. It may be good to have your ticket stub from your flight by which you entered China, and it may be good to have checked baggage on your continuing flight, but those may not be necessary. (We did have checked baggage. For us, at 6:30 a.m. with no line at all, we were through in 30 seconds.
There is a storage facility for your carry-on luggage, so if you are just out for the day (as we were) you don't have carry it with your. Cost was 60 RMB for two bags, a little under $10 US ($5 per bag), and we put it on a Visa card.
Also a nice feature at the Beijing Airport were what they called "Lockers" -- they are changing rooms, lockable, about six feet square. Free. Good for us, coming from Bangkok and going out into Beijing for the day, to put on extra layers of clothing. Weather was about 20 F with a strong chilling wind. If you ask, or follow the signs for "Lockers" you will find them. The changing rooms are portable (on wheels) so their location in the airport may change.