Common sense dictates that it is impossible for a goalkeeper to remain perfectly on his line while attempting to save a penalty. For this reason, it happens that even although the rule was added fairly recently (after the 2002/3 season I believe) it is rarely enforced.
Based on my own opinion and the opinion of other referees I have worked with, the keeper should be allowed some leeway in the form of a couple of steps, amounting to him being ideally no more than 2/2.5 yards from the line.
For a penalty to be retaken following the offence, the keeper would have to be taking the p*ss with his distance from the line, for example, over half way towards the 6 yard line. The reason that you only penalise under these circumstances is because you would be seen as overly officious and causing trouble for yourself as generally this rule is rarely enforced. So for me to bring a penalty back, it has to be very clear to everyone that the keeper encroached.
And for the record, I believe Benfica's 2nd penalty should have been retaken.