I've just created this site. Seriously all I've done is add a header image (1000x288) and one widget, and it's sent the sidebar to the bottom.
I've tried deleting both the things I've added, clearing my cache, and checking the "WordPress should correct invalidly nested XHTML automatically" box - nothing.
I've run it through a validator and got this, which I thought might mean a problem with the search widget, but disabling the search widget did nothing.
I've googled a lot of help pages, but they all mention things like divs not being closed correctly in custom code. I don't have any custom code.
Any ideas, anyone? I'd really like to get this site up asap.