Source: http://tsun2.bbs.coocan.jp/?m=listtop&p=2
(in Japanese)
Remarks: The new design is used for the K-01, K-30 and the new Silver Edition of the K-5. All older K-5 used the old design. It is also told that the newer 645D units are modified to adopt the new battery as well (and shipped with the new batteries with the new chargers).
The difference is the additional "D" contact, which appears in both the battery itself and a new charger with the corresponding new pin is required.
First of all, I suppose the P designation means "Plus". So, what is the new pin for? It is said in the above Japanese article that it is required as part of the newly enforced electrical appliance safety regulations in Japan, as it has been confirmed in the previous official Pentax announcement (Japanese).
Now, look at the above diagram, the old pins are "+", "-" and "T". Before we are going to guess/find out what the new "D" pin is for, what does "T" represent at the first place? Well, I once guessed before that it may be the centre (common) tap of the two cells so that each cell can be charged individually. However, a quick check to my BC-90 charger's printed label at the back revealed that the output rating as shown is 8.4V (at 0.4A), which simply means that the two battery cells are charged together in series. (N.B. The typical native unregulated output voltage of a lithium-ion/polymer cell is ranged from 3.8 to 4.2V). The following is a typical charger circuitry and design:-
Credit: http://www.bowdenshobbycircuits.info/page12.htm#lithium.gif
In general and on the other hand, quite some people believed that that T pin is for (direct and better) temperature sensing, which I have no objection on that supposition, but yet couldn't find any evidence for supporting this *guess*.
Now, let's think what the D pin is for. Actually, same thing happened for the Canon Lithium rechargeable batteries and there was also an evolution and change of the same. The newer ones are equipped with an additional "C" pin.
Without the C pin, there is no information and data about the status of the battery. So, I now know that the C pin could simply mean Code and it is almost certain that the D letter assigned by Pentax means Digital or Data.
This maybe part of the rationale of the new Japanese Law for why this is needed, i.e., to prohibit the use of 3rd party batteries, especially those with inferior quality, and to force more data communications (and thus checking and monitoring) amongst the camera, battery and the charger.
In fact, my Canon 5DIII reads the identity and data from the battery and also write number of shutter actuations per charge back to it. When the battery is used up, the counter is reset by the charger after recharge.