Both of the columns here are fluid and I've used 'background-color' for the background rather than an image. Have a look at the code below - the background colour is in the #rightbg div, which is 'position:absolute;' with 'min-height:100%;' ('height:100%' for IE).
The background div will resize in the same way as the floated right sidebar that's sitting on top of it, so it looks like it's one column.
I've given the body a min-width just so that it doesn't look stupid when someone makes the window really small.
Most of the code for this page is explained on the other layout pages, so please have a look there.