And the ie6/7 stuff isn't much to worry about, either - you can just use conditional comments to include a traditional CSS Sprite-based file if ie6/7 is detected.
Yes, it means more overhead, maintaining two versions of your CSS file, but the improvements in speed might be worth it.
IMHO one saved request isn't work doing the same thing twice. And MHTML (http://www.phpied.com/mhtml-when-you-need-data-uris-in-ie7-a...) doesen't come as a saviour either, as you have to duplicate all images in the CSS, which increases the size of the CSS significantly (if it doesn't you are probably better off using "old fashioned" techniques.
Assembling a huge sprite image and emitting offsets is harder. Still very doable though, of course.
Game engines has done this for almost 20 years.
I'm also wondering if, to correctly emit the offsets in CSS attributes in all cases, you'll require some manually inserted placeholders.
Such a shame.