I have strong feelings about the cabeceo: I LOVE THE CABECEO! I always teach it in my beginning classes, and make my private students practice it. As a cultural anthropologist, I can't imagine teaching people to do a dance without also teaching the appropriate cultural rules that go with the dance.
When I first encountered the cabeceo in Argentina, it took me a while to feel comfortable with it. I am quite shy with people I don't know (yes, I know I hide it well, but there it is), so the experience of making eye contact for a seemingly long time made me feel uncomfortable. I forced myself to look at stranger's faces, willing them to invite me to dance.
At first, I danced with anyone who was willing to look at me. As I got to know the other dancers, I found there were folks I did not want to dance with; and I could avoid them politely by not looking at them. Having been trained as a nice North American girl, I have difficulty refusing a dance partner who walks up and asks me to dance. Cabeceo gave me a sense of control over partner-picking that I never experienced in North America.
This is not to say that the cabeceo is fool-proof. This past February in Buenos Aires, I enjoyed going to La Nacional on Wednesday nights, as well as the Centro Leonese (Nino Bien) on Friday evenings: women were seated on one side, men on the other. We all had our glasses on to see, but you could see almost all the potential dance partners and angle for a cabeceo better. At one point, a guy smiled at our table, and all six women (sitting three deep) pointed to themselves and mouthed, "Yo?" He made an apologetic gesture, pointed at the one he wanted, and she got up to dance. The other five laughed and readjusted to look for another partner. That sounds chaotic, but I prefer that to being accosted by someone I've tried to avoid by not looking at their face all evening!
With patience, the cabeceo becomes easy to use and provides a lot of control for both partners to choose a dance couple for a tanda.
Someone on the listserve mentioned that it was inappropriate for a woman to cabeceo another woman. I don't think I agree with that. I wouldn't cabeceo another woman in Buenos Aires at a traditional dance, but I would at a gay milonga, where the rules are more relaxed. With women who are my friends, we use the cabeceo. In Portland, I rarely use the cabeceo women (or men) who I don't already know.
As a teacher, I have always felt that I need to be the model forcorrect behavior in the community. Because I know beginners get better much faster if more experienced dancers dance with them, I have always interpreted that as needing to dance with anyone who asks me (short of those whose grip hurts my body). However, perhaps I will do more good as a model of correct cabeceo! Remember, if I don't have my glasses on, I might not have ignored you: I may have just not seen you looking! Try again!