Updated for WordPress 2.8:
I had an interesting question from a client last night, wondering how come her gorgeous sidebars disappeared when she put widgets in them. Here’s the scoop:
Sidebars: Most of the time, the theme you are using gives you two options. First, you can just leave the sidebars without widgets and [...]

Q:  Why is my sidebar swinging below my content?
A:  9 times out of 10, it is the same reason: there is an unclosed division within an html/text widget.  Hang in there with me, I intend to explain this in English, I promise.
The Problem:
Your browser thinks that your sidebar belongs with the main content area.  If [...]