In contrast to the OP's example, Activity Monitor shows my MBP (Mountain Lion) has only 53MB used by Dock. And that's with about 60 items in my dock (though only about 12 apps open right now).
Wonder if his GB of Dock memory gets reduced after a simple killall Dock
That's obviously a bug. Try restarting OS X or the dock. Already had some memory issues with the Finder, restarting helped.