I noticed that the "fieldset" tag renders a rounded corner border on IE (it renders squared on the other browsers).
Some items (buttons, input boxes) are using the system visual styles by default - and in the default Windows XP/Vista themes, fieldsets have rounded corners. (Take a look at Display Properties, for example.)
If you assign any style to an
<input />, for example, it will lose its hover effects, gradients and other things too.