I think you should be pretty careful here! It's most likely that the wood shrank a little and the tight tolerances are holding it in. It's also possible that the varnish wasn't completely hardened when the plate was installed (sometimes the finish is a little thick in these areas which are not buffed) or there's something sticky underneath the plate).
If it is a tight fit and you try too hard to pull it out, you run the risk of pulling the finish up around the truss rod cover area. You don't want to do that!
So, I'd actually try the opposite route of what people have suggested here. Give the truss rod cover a light tap to see if it will loosen up. Put the eraser end of a new pencil on it and give it a few very gentle taps with a small hammer. If there's something holding it on that shouldn't be, you'll break it free.
If it's still too tight to remove, then you might try tracing around the sides with an XActo knife.
I think it's probably varnish holding that plate on. If the spray was a little thick in that area, the pressure from having the plate screwed down on it would have caused some adhesion over the years. This is probably also why goatfoot's Brown Bass cover didn't stick when others did (that's an oil finish instead of polyester or polyurethane). If this is the case and you don't snap the bond first, then you could well pull a big chip of varnish up if you pull too hard. The polymerized varnish will probably hold onto itself better than it can hold onto the wood.
Good luck,
David Fung
