Hey, found this topic when I was looking for a way to reliably clear the recents menu for most Mountain Lion apps.
I'm finding, at least in Mountain Lion, that the behavior of the "File < Open Recents" menu being in sync with the dock's recents menu, which is the same list as you get when you invoke that app's Mission Control, is completely inconsistent. I tried the above method for deleting that preference file. In my case, it was Preview I was trying to fix, so I found that same preference file for Preview, deleted it, and it had the effect of completley getting rid of the recents menu, but then when testing it by opening a new PDF document and closing it, it didn't appear in the recents menu at all.
Then I tried MadEyeMark's method--thanks! It seems this is the only solution that reliably works to get the open recents menu in the menubar back in sync with the Dock's. It seems that there is some kind of cache going on for apps in the Dock where it is keeping the same recents item list until you either clear the app from the dock or restart/log out. Kind of ridiculous that they can't get the list to update itself dynamically, and in fact I'm going to report this to apple.com/feedback to make sure Apple knows about it.
Hmm, intereting. While I was trying this, I was opened a PDF with Preview, cleared recents from the menu bar, and now it seems to update the dock's recents as well, dynamically even while Preview is open. So it seems just in certain apps the list get stuck occasionally, and you just have to clear them from the dock, and then it "unsticks" or whatever. Still, people annoyed by this should report it to Apple.
I like the general feature of having recents appear when you invoke Mission Control on a specific app, I just wish it was always consistent with the "Open Recents" application, because it tends to get really full down there, so I just wish it would reliably clear those when you invoke the "Clear Recents" command in the menubar.
Anyway, glad I got it fixed for Preview, at least temporarily.