According to Slashdot and Security.org, Kensington Locks are extremely vulnerable to very simple attack. With the most basic of tools costing less than $1, it"s possible to unlock the device that offers "security" to many thousands, if not millions, of laptop users worldwide.
Security.org report that using a bic pen and a pair of scissors, the protection from theft can be over come within 30 seconds. More worryingly, the procedure does not leave any damage to the cable / lock. As such, you would not be able to claim the $1500 that Kensington offer if a product is stolen whilst protected with one of its locks, due to the specifics of the guarantee.
So far, Kensington have not commented on the issue. One would suggest that you keep, as ever, a rather good eye on your laptop and any other products protected by their kit.