Well I did do one thing, that was remove most start up items. I think it appeared the next time I switched on the PBook after that. But there was nothing important on the list of items, just iCal, etc.
You may have accidentally dragged some text clipping or null area of a window to the desktop. Some applications allow you to drag and drop clippings to the desktop, like System Profiler.
It's easy to do -- think of how many times you drag something to a specific folder and accidentally drop them somewhere else. I do it all the damn time.