fieldset+legend padding-top is a mess in IE.
All padding-top values appear ABOVE the legend, thus pushing the entire fieldset box down (i.e. just like margin-top)
The easiest solution is to use <!--[if IE conditionals, and redefine the CSS for IE ONLY as:
fieldset {padding-top: 0}
legend {margin-bottom: 15px}
...yeah, I know, if IE conditionals are very ugly hacks, but the only way you'll get that space in the top of the box is adding it as margin-bottom to legend.
OR, you can drop the padding for fieldset altogether, then wrap the contents in a div block with its own padding/margin. Haven't tried it that way, but it should work, and it should be cleaner.