I have had the same experience as noted above.... use the icon to lock the screen.... then some time later come back and the screen is unresponsive to taps. Power down is different in that it suddenly powers down, without the dialog box, as noted above by casualt (4-7-2010).

I have also found, that if I do not use the icon to lock the screen, but simply close the eDGe, which also puts it in lock mode and turns off the backlight, that it wakes up fine every time.

So, I no longer use the lock icon....just close the device.