The calendar looks great in Firefox, thanks in great part to the use of curved corners using the -moz-border-radius property in your CSS.
That now needs updating to use the new border-radius property so that things will look curvy in IE9 and other CSS3 based browsers.
That still leaves IE8 and earlier with nasty sharp edges to all the boxes, it really looks so poor compared to the radiused corners.
I came across the CurvyCorners.js file and am having a play around with it - early results look fairly promising.
I will report back if I get it all working properly - it makes huge changes to the page's HTML and I need to work round the issues with storing date data as a div's id.