I use http://www.peninsulavisa.com/visa.htm. In the past I have FedExed my passport mid-afternoon one day, and had it back before lunch 2 days after that (ie less than 48 hours). If you live in the bay area it can be even faster (no FedExing time).

If you are going via Hong Kong, you can get a PRC visa same day there, and it's quite a lot cheaper - see http://www.ctshk.com/english/useful/chinesevisa.htm

You used to be able to get a visa "on the spot" at the border crossing at Lo Wu, but I'm not sure if that is still possible (but a quick google suggests it is). The only catch was that you had to pay in cash, in RMB - so be prepared and change money in advance.

Correction - US passport holders cannot get a walk up visa at LO Wu; most other nationalities can.