Basically when you press your shutter button once and you hear the camera's "click" sound, that counts as one.
Not to worry ..the D90 shutter is tested to 150000 cycles...im sure it will take you a long time to reach that number unless if you are shooting photos 24hrs a day, 7 days a week..
Don't worry, for hobbyists like us, we wont even come close to hitting the avg shutter lifespan! and yes, like someone mentioned above, the quoted lifespan is something like 80-100k so relax and just shoot
There are no solid proof a lifespan of a shutter. Mechanism things. can go haywire at anytime. . There is not death count for any shutter count. But there is such a thing as wear and tear. I have a friend owned a D3 and have to change his shutter at 55k count cos it fails.. there are some who had even hit 100K and still going strong.. So it is just depend on your luck ...
But whatever it is.. dont worry about shutter count. Just enjoy and shoot. That is what Photography is all about.
I think Ang Pow is explaining it the other way round. After I re-read his explanation, it makes sense. Instead of him saying number of times one depresses the shutter, he uses the illustration of how many photos one has to shoot before the shutter reaches ~ 20 000 (80GB worth of photos) , as he uses the file size according to his example.