Can an Android tablet serve as USB Host and be charged simultaneously through a single port?
Apparently it is possible to charge the Host-Device!  --> 
 http://en.wikipedia.org/wiki/USB_On-The-Go

Under "OTG Micro Plugs" it says that a USB OTG cable with a 36.5 kΩ resistor between Pin 4 (I suppose its pin 4) and Pin 5 allows you to connect a B-Device (Slave) and (!) a Charger to the Smartphone/Tablet.  

The Phone and the B-Device can be supplied by the external power source.

USB-Power Specifications:
http://www.usb.org/developers/docs/devclass_docs/batt_charging_1_1.zip (updated link)