Not a bad solution! Mine is definitely more basic, and has not been debugged or tested in browsers as old as IE8. A caveat though: if you're doing RWD, you may want to abandon full support for browsers as old as that or commit yourself to coding a lot of "if <= IE8" exceptions. Your site should degrade gracefully, but a lot of the techniques used in responsive designs are not supported.
I don't think pushing down the content is hard at all. If you take a look at the source of my example, I achieved it in two lines of jQuery. You simply grab the height of the open menu using a variable:
Example
var openMenuHeight = $('#menu.open').height();
And then apply that value to the content area (it must have relative positioning for this to work):
Example
$('#content-area').css('top',openMenuHeight);
If you look at the code of my example, it is a semantic unordered list system (as opposed to a tab structure) as well. I'm going to keep working on it, because I enjoy the challenge, but I'm glad you found a solution to your question!
[ed. note: designcouch last edited this post 12 years, 8 months ago.]