The GPS icon only shows in the task bar when an application is using GPS. There is the traditional route of going through Settings>Location & security>check/uncheck the Use GPS Satellites box.
I can't seem to find the Power Control Widget on the XOOM. I use Widgetsoid for power control on one of my phones, so I'm using that on the XOOM now, as well. With this widget, you can select a GPS toggle that can be right on your home screen. HTH