Sigh...then it's broken....it does some sliding, a few millimeters, on both left and right side...
I haven't dropped my magic mouse, and there's some very slight play in the top surface, in all horizontal directions. It's probably less than 1mm however, so unless your mouse shows significantly more looseness than this I wouldn't worry, especially if it doesn't hamper everyday use of the mouse.
Unfortunately the magic mouse is held together with magic glue rather than regular ol' boring screws, so you can't really take it apart and see if there's any way to tighten it back up again.
If you're really concerned about this I suggest you take it back to the point of purchase, tell them what happened, and ask if the mouse seems OK or not from their experience. You could also lie of course and say it was like that coming out of the box, but that would most likely damage your karma and I don't recommend it.