Last September, Pentax announced no less than five new optics that together cover every single current lens mount in its lineup, but of these one was particularly eye catching. The HD PENTAX D FA 645 Macro 90mm F2.8 ED AW SR lens is extremely unusual by medium format lens standards, because it includes image stabilization. (And the image stabilization is a little unusual for Pentax, because it's not in-body. Moving a medium format sensor to counteract shake simply isn't feasible.)
That stabilized D FA 90mm lens is shipping now, but to take advantage of its stabilization you'll want to apply a firmware update to your Pentax 645D camera body, because previous 645-mount lenses haven't been stabilized. The new firmware is available immediately from Pentax, and it adds one Custom function to the 645D, letting you choose whether stabilization should operate at all times when the shutter button is half-pressed, or only at the moment of capture to save battery life.
If updating your 645D firmware to use this new lens, you should also update to the latest release of Pentax's imaging software, Digital Camera Utility 4.