Just to start, I have searched everywhere for an answer to this and have found similar but not the same. It seams since the last WordPress.com upgrade (since I haven't had any issues before) whenever I publish a new post, my sidebar moves to the bottom of the page below all of my posts. I am on the K2 theme and have notices what might be causing it.
A couple of weeks back I noticed the new div element tags in the code editor for the posts, which I have never noticed before. If I leave these tags in, the sidebar moves, however if I go in and edit manually deleting all the div tags, it stays normal.
I am using the Firefox browser on a Mac, by the way. Has anyone else had the same problem and does anyone know how to solve it? You can take a look yourself at http://darjon.wordpress.com if you like.