Your problem is that your individual post pages have been truncated at some point recently: they are currently missing the body and html end tags as well as the all important footer div. It’s all important because it includes the attribute clear:both.

In standards-based browsers (e.g. Firefox, Opera, Safari, Konquerer, etc) you’ll notice the white background is stopping at the end of the righthand sidebar. If the content is longer than that sidebar, it will overflow the white background into the blue background of the body. This is because the right sidebar is the only fixed section of the page; the left sidebar and the content are floating. The clear:both attribute in effect restarts the fixed content of the page after the end of the floating objects. Floats can be powerful, but they can also give you one hell of a headache.

I can’t remember if your individual post pages had the same footer as on the main page, but I suspect so. At the very least you’ll need a div with a clear:both style attribute in your html code after the end tag of the sidebar div but before the end tag of the page div (the footer div has clear:both as one of its attributes already, which is why it does the job on the main page).

Keep up the good work!