I'm having exactly the same problem - it has happened many times, seemingly at random (although possibly when there is a slight movement of the laptop). The computer will always eventually go back to how it was (screen on and trackpad working) but it will take anywhere from a couple of seconds to over a minute. I have taken my computer in to get looked at by an Apple Authorised Service Provider (under warranty) and they were unable to find anything wrong with it, which meant that I had to pay for the testing (Apple apparently only pays if they can find an issue) and the problem still keeps happening as nothing has been fixed.
The article that Jason L has linked to unfortunately does not accurately describe the problem - it is nothing to do with the computer going to sleep (as it will happen in the middle of use) and also nothing to do with the computer being out of power.