There are maybe 2-3 weeks per year when I wish we had air conditioning, at most.
I live in one of the hottest cities in Germany (Karlsruhe) and I agree with ido. There's around 2-3 weeks where home air conditioning would really be useful. The rest of the time, temperature is controlled easily enough by opening windows at night and closing the shutters during the daytime while at the office.
Air conditioning for offices is a different story.
EDIT: oh but transit here should definitely be better air-conditioned: I don't think there's much you can otherwise do to make subways/trains/busses pleasant once it starts getting hot.
I remember I returned from my parents where I had stayed over the weekend and it was okay - to a city that was still glowing from the heat at 11 pm at night. I say that as someone who spent a summer vacation in the Emirates (that place has cooled swimming pools right at the beaches, and for good reason the Gulf water is not warm but unbearably hot!).
In general, if/when global warming picks up we have to make some major changes to all our cities. As they are now they are giant furnaces and heat batteries with all the exposed concrete and asphalt.