Magic trackpad locking up iMac

I am running Lion and sometimes for no apparent reason while using my magic trackpad the launcher window pops up even though I've not gone near the icon on the dock and the mouse pointer goes to either the top left or bottom left corner of the screen and even turning the track pad on and off or trying to use the mouse will not move the pointer. The only way I can overcome this freeze is to turn my iMac off and on again.
Does anyone have any ideas?
Thanks in advance.

I believe there is a selectable gesture that you can choose on the Magic Trackpad for Launcher. Do you have that set? In any event it shouldn't cause the cursor to freeze.

The gesture you mentioned was turned on, but I have just turned it off to see what happens.
Thanks for the suggestion.