In 1.4.6 and before, when the block a pressure plate rested on was pushed by a piston, it would remain in place if ta valid block remained underneath it. First two images show this behavior in 1.4.6. In snapshot 13w01b, any block movement under a pressure plate will cause it to pop off and become an item, even if the block that is moved underneath the pressure plate would be a valid block to place one on. Next two images show this behavior.
My testing shows this change also affects vines, ladders, snow, levers, and all types of rails, buttons, torches in addition to pressure plates. There may be other blocks I'm missing. The behavior of paintings and item frames seems unaffected by this change though.
Unsure if this is the new intended behavior of these blocks or is a side effect of the changes to pistons etc. The old behavior would definitely have some nice applications now with the weighted pressure plates and hoppers though (swapping fences for solid blocks and vice versa). Really, if the old behavior was not intended to persist, I don't quite see why weighted pressure plates were created at all.