I didn't try this directly but it's worth a shot. And looking at your code, it might be easier for you to try than me
float the #content right.
make a div in both the #menu and #content. And then style it something like:
Then position it where it will sit on top of the bottom border of the parent div it's in or dead bottom (with a height of .1px it probally will be barely noticeable).
Since I didn't try it I'm not sure if the div in the #menu will "see" the floated #content and visa versa but it's worth a shot.