I, too, may foray into the research aspect of PT. We already have two mandatory research rotations to complete but we can also complete an optional 3rd to replace a clinical rotation.
To answer your question, with the DPT expanding to make competent clinicians, theorist, and novice researchers you may not always have to complete a terminal doctorate in order to go into research. i also think it is program contingent. Each program has its strengths and limitations. Fortunately, I am in a program that harps on research(for the purposes of being a clinician and actually engaging in research). I would do my HW to see what different programs offer. As Dancer and goyo stated, some programs have combined DPT/PhD programs. That can be very enticing, yet you also want to consider what the PhD program entails. Some programs may have a PhD in Rehabilitation Science, Human Movement, Neuroscience, Physiology, etc. Thus if you have a specific research interest, yet the program only offers that specific area of study it may be more prudent to finish your DPT curriculum while keeping an eye to applying to PhD programs at another institution.
Finally, I know that a DPT student in my program just graduated/received their license last year and went straight into a PhD program at the same institution. So ppl are doing it. I imagine that it is not common place, but if you want to be an expert clinician and researcher then I don't see why you wouldn't be able to do the DPT/PhD or DPT then PhD.