It all depends on the width of your viewing window. If you grab the side of the browser window and drag it in, once it gets inside a certain size, the sidebar stuff drops to the bottom of the page to maximise the width of the forum itself. Needless to say, it seems that most (all) mobile devices are inside this cutoff, so the sidebar is always at the bottom of the page.
This is something that bothers me a little bit too. It's cool and all that the site is so well optimized for mobile browsing, but then the large resolution site doesn't look as good.
If you can somehow block the two flash banners located in that same right-hand column, the problem is lessened a little but the ugly empty space remains.