Folks, it's simple. The Evo's screen is known (as far as has been reported) to have a tempered glass (and a plastic camera lens cover, why people think it's glass is beyond me, it's so thin).
If you recall from grade school Earth Science, glass has a hardness of anywhere from ~5.5-7ish on the Mohs scale of mineral hardness (which is a scratch test ranking of minerals). This means that Glass will be scratch proof to most soft metals (iron, copper, nickel, lead, gold, and most types of unhardened steel), plastic, and harsh language. So your keys, razor blades, and paper clips are likely not to scratch the screen.
This however does not comprise the entire universe of things that will come into contact with the screen. What commonly occurring substance will scratch the screen? SAND. Sand and other forms of debris will easily scratch glass, and if you don't know what scratched your phone, I have $100 riding on it being a grain of sand.
For example, it's not unusual for sand and other debris to be caught in the pockets of jeans during the stone washing process (I know I've had to wash jeans with the pockets inside out several times to rid some of them of all the grit and sand).
Unless they made the screen with diamond, there is no way the screen will be scratch proof, especially to sand. The scratch test video mentioned is nice because glass is a significant step up from the plastic screen of the Palm Pre and most older phones, and glass will resist and significant amount of scratching from keys, razors, and such.
