I think the ones who look at your hairline, are the ones who are going bald themselves, or know that they will eventually.
I remember watching all these movies, like Forest Gump, Last of the Mohecans, 3 OClock High, in which the main actors have lost a lot of their original hairline. But I NEVER NOTICED, I mean I never thought twice about it, until just last year when I realized I was beginning to lose my hair as well. Now I notice, and I am that jackass that tries to cop a stare at somebody worse off than me, when they aren't looking. Usually I am thinking "Hey, he's balding far more worse than I am, but he still looks 'okay'." But I figure if I am looking at hairlines because I am aware of my situation, than those looking at my hairline are aware of their situation as well. Just an assumption, though.
And girls... I just don't know. I think if a girl is checking out your hairline, it *MIGHT* mean she considers you interesting, and is weighing the full package. I really do agree with the idea that the right girls just won't care. My two best buds that are far worse off than I already have awesome girlfriends, and are getting married this Fall. Bastards. =)