supposedly a major raptor refresh is due out around next summer according to someone who knows someone who works there, a relative if i remember right. 15k rpms being the spec that was mentioned. pmr is a possibility for platter density increase, but whether that will result in a usable capacity increase is a different issue, as they may only decide to stay with the 36, 74, and 150 capacities, for whatever reason. other additions then may be the most likely necessity of sata300, as sata150 would probably be a bottleneck if they include the above specs.
either way, if and when they are released around next summer with at least 15k and pmr, then they 'should' easily be the fastest performing mechanical hdd again on the desktop, possibly trailing only ssds in performance, but not $/GB if current prices dont drop noticably.
but, i agree, it would seem the main reason wd has been pretty stagnant with major raptor updates and pricing, is probably because of the lack of real performance competition on the desktop. that has changed though as of the last few months in a few benchmarks, but before then, raptors really had no desktop competition to speak of.