Stuck Key Detected - even with keyboard removed!!

I've got a Dell Inspiron 1501 Laptop.  When I switch it on it produces the most deafening series of beeps.  The Dell Diagnostics CD says:

• Error Code 2C00:041D Stuck Key Detected
• Keyboard is Continuously transmitting a scan code 31h

It would appear that the left cursor key is stuck, as the only way I can get into windows (using an external keyboard) is to type my password backwards! instead of 'password' I type 'drowssap' The cursor is constantly moving left so by typing the password backwards it comes out the right way round.  WEIRD!

Now the weirder part.  I took the cover off and disconnected & removed the keyboard and trackpad.  But the problem persists: it seems this key is constantly transmitting, even without the keyboard!!

I've cleared the CMOS, left it unplugged without power, battery, CMOS Battery for 3 days.  I've flashed the BIOS, changed the Memory.  Still the same problem.  Keyboard has been disconnected all this time, but getting this constant Key signal.

Where is it coming from if the keyboard is disconnected??  Any ideas on how to fix this?