This is one of the few "SDM Only" lenses with a "KAF3" lens mount, but its reliability is still in question for a long time since marketed, for a long time already. Here is yet a latest user report on the AF (SDM) failure of the lens:-
http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=36075191
In fact, there have been quite a number of previous failure reports, by different users/owners of the lens, for example:-
http://ricehigh.blogspot.com/2010/05/focusing-issue-with-da-17-704-sdm.html
One of the above users has actually switched, he sold all his Pentax stuff except the 3 FA Limited, including his new K-7 body which has just been purchased in April, just owing to the SDM failure case of his DA 17-70, which was his standard zoom that he purchased together with his K-7.
The above story once reminds us that the never-cured SDM issue and the negative attitude of Pentax/Hoya (actually lack of remedial action and response of any!) actually hurt Pentax (and of course the victim users, needless to say). People will not wait for a solution from Pentax anymore, what they do to resolve the problem by themselves is to switch! :-( (And they voted with their feet!)
Once again, as a timely reminder, I still encourage you to participate in the following SDM petition, although I bet it will be useless and never receive any attention/response from Pentax Imaging/Pentax/Hoya at the end of the days! >:-{
The SDM Petition Online