I cannot left click on any items in inventory, or in chests or furnaces. I can right click on items, but can't drop them once they've been picked up using a right click.
Noticed this immediately after the update in a world I'd been playing for some time. Created a new world to see if the problem persisted and sure enough, the same thing happened; I can't left click anything I pick up.
Not at all sure what steps can be taken, as I can't recall a particular action that caused this.
UPDATE 10/27/12: Tried using a usb mouse instead of the trackpad. Left click still does nothing (can't pick up/drag&drop/etc), right click works, but I can't drop any item.