In the summer, I never wear pantyhose. It's simply too hot and miserable to be in nylons (you may as well wear pants). I actually think nylons look more professional, but I don't care!

I see more women without nylons than with them. I don't know if there's a rule about it, though. I think some firms have dresscodes that encourage wearing nylons or at least have some guidelines in place that younger lawyers could check if they were concerned. In myexperience, it's a combination of things that makes one look unprofessional, though. If you're in a nice suit with professional shoes but aren't wearing nylons, I think the overall effect is still professional. But, if you're in a short skirt and open-toed shoes without nylons, then the nylons tip the (alredy tipped) scale further toward an unprofessional look. That said, I personally don't wear nylons in the summer.

The ABA Journal has a post on this and the end result seems to be that "bare legs are common even on conservative Wall Street and at business events."

On the West coast, I never see women wearing hose, even when wearing suits, in court or in high powered meetings.  In fact, if you ever do see one you know that they are from out of town.