When our dogs were puppies we used a virtual fence as well - the collars emitted a sound as the dog approached the 'fence' - if it continued to get closer the sound got louder and the collar gave a very mild shock - if it still continued the noise got louder and the shocks got stronger.

Neither of our dogs ever got more than a mild shock and in a matter of days simply kept away from the virtual fence, in just the same way that the horses keep away from the electric fencing whether or not it is switched on.

I don't see the use of collars to restrict dogs to a specific area as a problem - the sound/shock is administered immediately and dogs quickly associate the shock with the sound and learn that backing away stops both.  Over 12 years later they will still only go down the river bank into the river if either my wife or I are with them.

If you are going to object to the use of shock collars in this way then you also have to object to the wide use of electric fencing to restrict the movement of all sorts of livestock.

The real problem in my opinion arises when the shock is administered manually and is open to abuse and/or misuse where a shock is administered as a punishment rather than being consistently used to stop certain behaviour.  It is very difficult to see how a shock collar can be used to reinforce good behaviour and if the pain is associated by the dog with the owner then this must make positive training even more difficult and would certainly ruin the relationship between the dog and the owner.