Although most emergencies aren't ones in which calling 911 from your HTC EVO need to be kept a secret, there are some life-threatening circumstances that make discretion absolutely essential. If you're hiding from someone who's trying to harm you, for example, the last thing you want is to be found calling for help.
Unfortunately, a default setting on the EVO could be the reason you're not able to stay safely hidden until help arrives. Because unless you change it, every call to 911 will result in what may as well be an alarm. G&E reader John called it a "freaking warning noise," which is definitely not what you want going off when you're trying to hide or run away.
To make sure that your EVO isn't the one ratting you out when you call 911, tap Menu -> Settings -> Sound. Scroll to the bottom, press "Emergency tone," and select the "Off" or "Vibrate" option from the pop-up menu that will appear.
Even if there's a legitimate reason for "Alert" to be the default setting (and I'm sure there is), a call to 911 is still a call to 911. Accidental or not, the authorities are going to know about it and will call you back, regardless of whether a loud beeping freaked you out first. So wouldn't you rather explain to the police that the call was made accidentally than have the police call your family to come and identify your body because the EVO told the killer where you were hiding?