Some of this has been covered before, but I'm not sure all of LSCman's info is accurate.
First off - on the 2002 Focus at least, I think the models with the OEM tach had ABS and traction control (basically the SVT Focus), but I'm not positive of that. You could pull the bulbs from the cluster, but I don't think there was a way to re-program the cluster to have the tach without the lights. Newer models might have allowed this.
Also - I have never before heard that it was illegal to swap used clusters between cars. It has always been illegal to roll back an odometer to show the car had 60K miles when it had 100K miles. I think it is legal to put a 60K odometer in the car and sell the car as long as you sign paperwork saying the car had 100K actual miles on it and the odometer reading was 40K low. Some buyers might not like this.
You might have a similar problem with the dealer. I think most dealers CAN reprogram a used cluster - in fact I think earlier in the thread people paid $150 to $200 to have this done. For obvious reasons, you might have an easier time getting the dealer to reprogram the cluster from 60K to 100K miles to match then car rather than from 180K to 100K to match the car. The dealer is taking your word for it - but really, the law is against modifying an odometer to fraudulently sell a car. (i.e. if I install a new cluster in a 100K mile car without reprogramming it and I want to pretend my car is brand new, that is fine as long as I say it has 100K more than indicated when I try to sell it).