Initially, I actually though that maybe Merc did have something up their sleeves. I was under the impression that they were going to release their new car in the final week of testing.
However, given that they are now bringing their new car to the next test (ie. they only missed 1 test, which isn't a huge deal), I am now beginning to think that they simply didnt have their 2011 car ready in time for the first test...I don't think there is any top secret gizmo that they are hiding.
However, in saying all of that, lets wait and see what happens in the next few days, as that will at least give us a better indication of just how good Merc are.