The only thing that I would recommend is that you adapt it to follow our html guidelines - http://doc.silverstripe.com/doku.php?id=html or follow them as closely as possible. Just like for naming the divisions of the site. Its not important but we trying to set up a SilverStripe standard way so that everything is super modular and flexible!
Looks pretty good I'll download it and test it out now!
can't you just put the images on a white background? They don't even need to be transparent unless you think people will be changing the background of the content area. I would also add this to the top of your layout.css so that when the page is short that bluey background comes through.
// to the top of your layout.css file
Will's idea about the background colour is good. That works well.
The IE hacks for alpha transparency I have found are dodgy - it relies on DirectX for the alpha transparency which sometimes crashes in IE6, it's best to avoid using hacks for this unless you absolutely necessarily need to.
You could just save the PNG icons as PNG-8, instead of 24. This stops the alpha transparency problem because 8 uses indexed transparency instead like the GIF format.