@Anonymous Coward [Easily Sorted]
"To be honest this is only a serious concern if you've registered your printer & serial number..."
1) Government has document they want to trace. They extract the serial number + date / time information.
2) They present the serial number to the manufacturer who tells them which supplier bought the printer.
3) Government contacts supplier with serial number to find out where it was sold.
4) Repeat 3 as required until at end of chain.
5) Your "unregistered" printer is now traced.
In practice, this won't take very long at all as all suppliers track inventory with the serial number. Oh, and in the future imagine if you have to present your ID card to buy a colour laser printer. In which case, they'll record the S/N on the central government database.
"*and* you're printing stuff that you might not want traced back to you."
Maybe when you bought the printer you had a government that wasn't so control-freakishly oppressive that you wouldn't... but nowadays?
p.s. Surely, the answer is to print on yellow paper?! :P