How do rounded corners affect usability?

How do rounded corners on buttons and containers affect usability? Do they make buttons look more clickable? When used on other non-button elements, do they make the site look more friendly? I'm looking for some scientific tests on their effect.

Rounded corners are very time consuming to do in old versions of Internet Explorer. Many websites, such as Digg, just use border-radius: 0.4em on browsers that support it and leave bad browsers with sharp edges. Did they purposely do this to IE because the time needed to implement rounded corners in IE was not worth the usability increase?