My curiosity was piqued by this Kottke post, so that a quick search turned up these awesome photos of happy inhabitants. More after the jump...
The folks at Opium Museum speculate that they were possibly taken by famed California photographer I. W. Taber, whose career was ended by the '06 quake and fire, during which his studio, gallery, and negative collection were all destroyed. From the Opium Museum's Flickr account that these images are posted in:
The photographer obviously had built some rapport with his subjects. Opium dens were dimly lit and so the photographer would have had to use flashpowder -- newly invented in the late 1880s -- to illuminate the scene. Some photographers of this period took photos of opium dens by barging in and taking a single flash photo before running back out. The result of such photos can be imagined... This series on the other hand gives us an idea of the socializing atmosphere that could be found in these places if the visitor was on good terms with the opium smokers.
Here's a confirmed Taber image, "White Women in Opium Den, Chinatown, SF," from 1892: