I have an issu with my css, I hope someone can help!

I have a sidebar showing news and twitter messages.
This sidebar has a background-color and 2 rounded corners.
I want the height of the sidebar to grow if the rest of the page grows.
But at the moment the side-bar has a float:right;
and height:100%; doesn't seem to work on a floating div.
If I give sidebar a position:relative; and a left:660px; without the floating, the height:100% works and the background of the sidebar does what I want.
But then the content drops to below the content of the floating div next to it. Which - of course - I don't want.

here's the link:

Any advise?