I bought my MD just a month ago and this had happened to me a couple of times. I still don't know if it's happening only to me and I should take my mobile to the service or is a problem all MD have.
You must have noticed that the screen turns black as soon as you take the mobile to your ear when you called someone and you need to talk right. If you take it back far from your ear the screen turns to the call options where you can see the speaker option, the hung up bottom, etc... Well, sometimes, when I finish a call the screen stays black, the four bottoms below stay lit and theres no way to go back to the home page or turn it of or anything. I have to take out the battery to completely re-start it.
Has it happened to you, is this normal, I guess not but I mean, should I worry and take it to the technical service or is something MD has to fix with a OS update?