If that photo of the Ho Chi Minh City buzzers were over here in germany, I am pretty sure you have all the govermantal agencies at your front door trying to explain to you why that kind of buzzers can't exist...

The role of the buzzer - at least for me - is a presence indication device. Most of the time appointments with friends are made via phone/jabber/mail/... Arraving at the opponents house requiers a notification of my presence at his door steps: the buzzer, a cheap alternative to a SMS

Where I live, in New Delhi, India, buzzers are ubiquitous! You cannot dream of having a front door without a buzzer. They are also sold in a variety of tones, tunes, etc. Maybe because buzzers existed a long time before mobile phones, they are still used...memory is hard to get rid of! And they are good security as well.

I like the relative flexibility of the Vietnamese solution. Want to change a name? White wash whats there and hand-write something else. No such thing as the fonts not being supported.

Would a chalk board work in this situation? Or too easy for anyone to change?

First of all, very impressive photos - love them.
In fact, one need buzzer at least in order to allow people who dont have one's phone can catch him. In the second place, buzzers are much more comfortable in that sense that you no need to care about where you are at home in order to him that someone has came in. In the third place, it is easier to press one button instead of typing few digits on phone, then waiting when one will responding to your call, saying him them you are near his door etc.

Speaking of the relationship between one's door and mobile phones, I love this photo of that ken anderson took as part of fieldwork in Ghana: http://www.flickr.com/photos/kenanderson/6505604/

It's an interesting aspect of identity. How are we identified when official street names don't exist, or our home location is transient. There's a lot of milage in SIM cards as forms of indentity.