Hey Ben,
One other reason (which I am guilty of) is the screen resolution of the viewer.
Mine is set for larger screens ao if someone has an 800x they get the sidebar after the posts.
Since only about 4-5% of my readers have such small screens, it isn't a major consideration for me.
Joe
 

I'm surprised screen resolution would push the the sidebar after the posts. I figured it would just end up causing horizontal scrolling (which is usually crappy too).

These days, very few designers do things for 800x600 anymore, it's more common to go for a minimum resolution of 1024x768. But it's definitely worth taking into consideration.
 

Ben, I had the exact problem you wrote about with my Google Search Box.

IE doesn't recognice the code that Google gives for shortening the box to get it to fit in the column. And that caused the whole right column to fall to the bottom of the page.

You'd think that an operation as big a Google would put out IE compatible code!

Firefox had no problem. I only caught it because I opened the site in IE for something else (I don't remember what now.)
 

Chris - that's a great example. I've emailed at least 3 or 4 people about broken sidebars where the Google search was the issue.