FSP's recent post got a bunch of comments regarding the correct form of address for professors. The academic folks were complaining about being addressed too casually by their students.
In my undergrad geology department, we were told to address our professors by their first name. Same thing with my much larger grad school department. But I just conferred with another alumnus, who said that in his department, the professors were always "Professor Lastname". So maybe the lack of formality is a geology/small department thing.
I just had a mini-addressing issue earlier today, when I was trying to get some information from an outside authority. I had sent an e-mail to her several months before ("Dear Ms. Lastname") but she didn't provide any hint in her responses. So I wimped out and responded to the ancient e-mail chain rather than starting a new greeting.
Environmental consulting is a pretty casual business. I've always been on a first-name basis with everyone else from the first handshake, whether they were clients or regulators. The only exception is when I'm dealing with the public, such as when I'm requesting access for sampling.
I understand the impulse to impose a certain formality on your students. When I was a TA in grad school, my labs sometimes felt like they were on the hairy edge of control. But I don't think that being casual in your form of address necessarily translates to being less respectful.