Have all updates been applied? This happened to quite a few people when the phone was first released and the updates since then resolved the issue. However, if you plan on rooting you may want to do that first, then flash an updated stock ROM. Current sw version is 2.08.651.2.
There were two updates released pretty close to each other. Once people applied them the problem was resolved and the os upgraded to Android 2.3.4. The thing is, the bootloader went from 1.4 to 1.5 as well and the only way to root is to use HTC's method. If you plan on rooting you may want to read through the Rooting section of this forum for more detailed information. If you are certain you don't want to root then go ahead and apply the second update.
If you are just talking about the phone icon disappearing from the lock screen then this might not be exactly what problem you are describing. To fix the lock screen issue check out this thread.
If it is something other than that then maybe it has something to do with the mirror app not showing up in the app list and you can still search for it.
yes, i was able to resolve the issue with the two upgrades.
the first one got rid of the phone completely and the second upgrade came thru a few hours later. Once i upgraded, the phone popped back into the lock screen and back into the apps.