Strange (unfamilar screen) on my Galaxy Tab? I have a Galaxy Tab 10.1.  It was powered off and charging when my nephew picked it up and turned in on.  When he handed it to me on the screen was a graphic of an open box, the android logo with the words downloading.  

It has been in this state for 4 hours now and I cannot switch the device on or off.  Please help me understand what has happened and what I should do.
Your device is in Download Mode. I don't have that device, but if the battery doesn't come out, you will probably need to use ADB and send an adb reboot to get it out of Download mode. If you don't know how to use ADB, then you can just let the battery die. I don't like that option though because the same image will be on the screen for an extended period of time.

Does the screen look like this:


To use ADB:


Download the USB Drivers from Samsung  
Download the Android SDK  
connect the device to your computer via USB cable
open a command prompt where the Android SDK was installed and change to the platform-tools directory. for example, my sdk is installed at c:\android-sdk so I run the following:

C:\android-sdk> cd platform-tools


then try to reboot with adb:

C:\android-sdk\platform-tools\&gt; adb reboot


if that doesn't work try the following command:

C:\android-sdk\platform-tools\&gt; fastboot reboot