The first study to indicate that chemical signals play a role in attraction was conducted by Claud Wedekind over a decade ago. Forty-four men wore the same T-shirt for three days. They refrained from deodorants and scented soaps so they wouldn’t interfere with their natural smell. Women then sniffed the shirts and indicated which ones smelled the best to them. By comparing the DNA of the women and men, the researchers found that women didn’t just choose their favorite scent randomly. They preferred the scent of man whose major histocompatibility complex (MHC) -- a series of genes involved in our immune system -- was most different from their own.
An interesting exception to the MHC attraction is for women taking the pill. Wedekind found that pill-takers responded in almost the exact opposite manner than would be expected. Because the pill tricks your body into thinking it is pregnant, it chemically alters your sense of attraction. Instead of finding the scent of genetically dissimilar men attractive, women on the pill found the scent of men with MHC’s similar to their own to be attractive.
Note that picking a mate whose MHC is different from your own is important in reproduction--couples with similar MHC suffer more difficulty in conception, higher rates of spontaneous abortion, and underweight babies (and possibly other effects of inbreeding) (see also this psychology today write-up on smell and attraction and MHC detection/compatibility). So to find a mate with whom she has high chances of successfully bearing children, a woman needs to be attracted to MHC-dissimilar men.
The Time article actually went one step further, suggesting that use of birth control could be partly responsible for high divorce rates. E.G. a couple meets while the woman is on the pill, and she is attracted to him, and then they settle down to have kids, she goes off the pill, and suddenly he's not the one for her. I guess to confirm that we'd need research on how many divorces do and do not involve going off the pill, and with what sort of correlation.
But anyway, this is all very interesting, and may call for the modern woman to apply a technological test to see if she is in fact MHC compatible with the man of her choice. Or she can take a break from birth control in order to have a "Sniffing" date, but unfortunately none of the research reported how long a break someone would have to take for the instinctual attraction to reverse back to normal.