After some investigating, I'm not able to reproduce what you describe.
Could you possibly post up or send me a test case for this?
I just encountered this bug (rev. 2454) when trying to get dimensions of a table inside an iframe contained in a hidden DIV (the whole structure is actually html>frameset>frameset>frame>html>body>form>div>div(hidden)>iframe>html>body>table but after debugging with Firebug I *think* the problem lies with the document inside the iframe).
I can't give you an example right away since the whole page&code are a bit complicated, plus I am in a kind of a hurry to finish this right now, but I could try to reproduce the bug later on a test page if it helps.