I am not sure what the "DC" actually stands for in the official marketing media article of the new DA 18-135 DC. But typically DC motor just refers to Direct Current Motor after all in technical sense.
But then the above press release also mentions about its "SDM autofocus operation" (but the lens now has no SDM designation btw), so now I take and (re-)understand SDM just only means Silent Driving Motor then, in contrast to previously told long form of Supersonic Driven Motor or whatsoever / whatever you or the Pentax marketing guys like to call it!
Below maybe the patent and detailed design that used in the new lens for that "DC" "ring-type" AF motor, have a look, it is not too difficult to be understood if you really want to study:-
http://www.freepatentsonline.com/20090323208.pdf
In short, it is just a "conventional" DC Motor with magnets arranged in ring-form, continuously and in cyclic, which is yet somehow unconventional then, as those permanent magnetics are now arranged "outside" the core/coil of the motor (in certain sense), but not inside in a self-contained unit and then the self-contained motor is used to drive anything "outside" as desired by/thro./via gears. As such, this new DC motor is simply nothing Ultrasonic and the working principle is totally different.
I would say it is a very clever design with low and old technology used. Let's see if it will perform and if it would be more durable than the old SDM motor units, which are proven to be rather vulnerable and slow which are just helpless, frankly speaking.
Putting aside the speed/performance issue, the other major concern is about its AF accuracy. My judgement is that if the motor has minimal inertia, then its focusing accuracy will be good. Otherwise the servo control and algorithms of the AF must be designed and written to be very good for the control model for a targeted/desired stop-down (curve) . The old SDM is proven to be not accurate for the stopped position and thus focusing accuracy is affected and sacrificed (which is actually worse than the old body-driven screw-type AF). Now, there is hope in the Pentaxland again! Let's see if this cleverly re-designed motor using the most primitive and old technology will work, or not! :-o :-)
Anyway, last but not least, I feel happy to see Pentax/Hoya really have done some R&D of their own. And most importantly, to realise their R&D results and put them into their new products! :-))