Recently purchased the EVO Shift. Woke up this morning and the screen was black, with a tiny picture of a phone, and an exclaimation point inside of a triangle. Phone was unresponsive and had to pop the battery to reboot it.
It really can be several things - it could have been performing an update and failed to reboot properly is the most usual reason for it. As long as it doesn't reappear after you pop the battery don't worry about it too much. If there is a program installed that is causing a memory leak that symbol may reappear and then you would need to see which program MAY be causing this.