Absolutely, excellent point. IE9 is promising to be an excellent browser. Once it comes out we should absolutely encourage people to upgrade. The day PIE is obsolete will be a very happy day. :)
Believe me, I have a lot of sympathy for this viewpoint. We don't want people continuing to use IE, so it seems logical that we should make their experience "lesser", using CSS3 for browsers that support it and falling back to flat square boxes in IE, and that will force them to change browsers, right?
Unfortunately, my experience over the years has brought me to the conslusion that this assumption is incorrect. If an IE user comes to your site and it doesn't look pretty, they're less likely to switch browsers than they are to just go to your competitor's pretty site instead. It sucks, but that's the reality, and that's why almost nobody gives IE a totally degraded experience.
Instead, web developers use all sorts of tricks of the trade (sliding doors, complex overlapping sprites, empty divs positioned to corners, etc.) to "fake" the features that CSS3 gives us natively, solely because they are forced to support IE. That's what PIE was created to change. It's not about giving IE users a better experience (most sites already do a pretty good job of that already), it's about making the web developer's life easier so they can deliver that same experience with less code and fewer images.
Thanks for the feedback! This demo isn't intended to be functional (clicking the tabs won't do anything, that requires JavaScript code which is out of scope for this demo), it's just there to show an example of how you could style things using CSS3.
I can verify there is a bug in IE8 where there is an extra gray line at the bottom of the active tab. I will attempt to fix that ASAP. It works properly in IE6 and IE7.