The advantage of this system is that it reduces the risk of having your PIN number stolen by keylogging software. Keylogging software can record what you type and email it to the outside. Keyloggers could be part of spyware or viruses and be used by criminals to steal account numbers.
This PIN Guard feature is similar to HSBC's Bill Pay Security Key.
Keylogging software targeted against these systems could still potentially break this feature. However, it should definitely make it more difficult. Also, if you want to make it even more difficult (or are very paranoid), do some fake clicks (outside the keypad) as you enter the numbers. This would make it even harder for criminals to decode your number from the keylogging software records.
As I mentioned in yesterday's post, ING Direct's extra question that varies with each login doesn't provide much extra security. I'm glad ING is adding PIN Guard, and I hope other banks follow along.