I am using an Intous 4 and AE CS4. When I use either the zoom shortcut on my keyboard or on my tablet as soon as I touch my pen to the tablet it Zooms out with each touch until I change the tool. The only way I have figured out a work around is to hot key a tool to toggle off the zoom. But there has to be another way, maybe I am just not in the habit of doing that, but everytime I forget it is frustrating. I seem to be zooming in and out more than working.
Check your Wacom system control panel. You have defined something there. Also update the driver if necessary.