Two of my coworkers have asked me this question. One is dating someone but is a generally nice woman-charmer, and the Q was asked while mixed company was discussing "what makes girls pretty?" The other guy asked me that question, and then proceeded to ask me out an hour later. I'm curious if I should be expecting this from guys in the future, and if they ask... what I can expect!
eta: I don't wear make up, and I don't hide that from anyone. I don't go around bragging about it, but when asked I don't act embarrassed. It just seems curious that they ask, as if it really mattered a lot.