I actually was going to make a post about this too. I was following the screencast about designing your own WP themes(number 26 & 27). I can’t seem to figure out why it is doing this. I don’t have a live site though. I’m using a local WP install.
I did notice that when i add a div around the post area that it drops down, but when i delete the div the header is still dropped down like this.
I only noticed this in FF3 and in every other browser this doesn’t happen.
Are you using FF3? did you test it on other browsers?