Vague accusations are difficult to confirm, deny or explain. Now, with a report by Richard Pollock of PJM, the charges become specific:

Both sources, one male and one female, worked at the time – mid-1990s – for the governmental affairs department of the National Restaurant Association, as did the woman.
According to both sources, Mr. Cain and the woman had been with a large group for a long evening of food and drink at the Ciao Baby Cucina, a restaurant near NRA headquarters in downtown Washington, D.C. This was a normal routine, as the trade association worked with the food and beverage industry. Afterwards, Mr. Cain allegedly took the woman by taxi to his apartment, where she spent the night and woke up in his bed.
The female source told PJ Media that she witnessed the woman entering a taxi with Herman Cain.
Neither source has direct knowledge of what occurred at Mr. Cain’s residence, but several days after the alleged incident, the male source witnessed the woman returning to her workplace “distraught.” “She was very upset.”

I have contacted the Cain campaign to seek an official response.