This was a little harder to find than I expected:

http://www.ncbi.nlm.nih.gov/pubmed/16555779

"These findings demonstrate that female facial appearance holds detectable cues to reproductive health that are considered attractive by other people."

Found via http://asutriplehelix.org/node/99 which also cites some other related papers

EDIT: I really, really enjoy finding cases where something I assumed to be true is questioned on the grounds of scientific correctness. It is always a wonderful opportunity to bring my thinking closer to reality, no matter if my assumptions are true or false.