Just got back from my local Sprint store with this exact issue, unfortunately they won't have any replacements until tomorrow.
At first the rep called it physical damage and that they couldn't do anything. Luckily I looked at their two display units while I was waiting and noticed that one of them had the exact same hairline crack. Upon questioning the rep about how the display unit had the exact same issue as me if it was my own fault, he was quite surprised to see the same issue. He brought a tech out to look at it, to which the tech called it a "weak point" and said they'd exchange it as soon as they got some more Evo's in.
To sum up, if you have this issue and want an exchange and the Sprint store is claiming that it is physical damage, check their display units in store. I probably got lucky with this but definitely check.
I'm going to go out on a limb and say that this IS an issue with a number of EVOs, probably a small number and dependent on how the power button is pressed. HTC could have eliminated this issue by simply making the power button stick out just a bit more so that it would take less force on the phone casing to press it.