To begin with, I don't think I'm like many women (although posts to this response could prove me wrong, who knows!).
If I ever entertained the idea of hiring a nanny, my top priority would be to find one that loves kids, is intelligent, has training and experience in dealing with early childhood developmental issues, and someone who could follow my rules. Looks are not even on the list, unless of course she is exuding constant sexuality that would interfere with her ability to take care of the kids (in other words, she's constantly acting inappropriately around the kids).
If she's hot - fine. If she's not hot - fine. It wouldn't bother me in the least and I certainly would not feel threatened. In fact, it might even be fun for my husband to be able to brag to his friends that he has a hot nanny. We'd both get a kick out of that! Both of us are very secure in our relationship, both of us work closely with members of the opposite sex (some of them hot too - oh my goodness!), and personally it wouldn't bother me at all. Of course if the hot nanny tried to seduce my husband, the trick would be on her because he wouldn't go for it and he'd fire her.