I think you’re experiencing something called a float drop. Try reducing the width of either the featured-posts and featured-properties ids or the sidebar id. You could also try increasing the width of the div box holding both of these.
I think the reason it’s being dropped is because IE handles css differently than FF and Safari, and unfortunately, it’s IE’s fault. It’s adding a couple pixels that aren’t supposed to be there. As for why it was working before and not working now, I have no idea why that happened.
good luck.