They are not released yet. A few places have just started listing them, Computers4Sure, TechOnWeb to name a couple. They are taking advance orders, at least for the 171P. I am eagerly awaiting this monitor and to hear what people have to say.
I've seen good reviews on the 170T, and the 181T and some early previews of the 171P have been very favorable.
I called SamsungUSA two weeks ago to ask about the 151P and 171P. They told me they would start to ship in the middle of this month. They both have a RT of 25ms. The prices I have seen will put the 151P around $550 and the 171P around $980.