Mouse over the links. These is a 20px padding on the top of the fieldset. When you mouse over the link, IE will appy the 20px to the bottom of the fieldset also. Anything that triggers IE to rethink about its layout will remove the extra bottom padding.

Full Width Example

Resizing the browser window will fix this

Centered Example with 50% width

Resizing the browser window will fix this too

Centered Example with fixed 500px width

Mousing over the "Try again IE" link will fix this one

Try again IE