I find this interesting. I just finished reading a design blog the other day that pointed out that there is definately a false bit of logic going on in web design circles.
That falsehood is that web sites should look the same in all browsers. Let's be honest, IE 6 was released in, what, 2001? It is nearly a decade old. It seems pretty absurd for clients to expect support for a 9 year old system that was deficient and buggy upon release.
I no longer support IE6 on my personal web projects, and this is not at all an uncommon posture to take nowadays. Check any Gawker media blog, you can't even comment if you don't have higher than IE6.
Now, some clients have the need for IE6 compatability. For instance, my workplace is locked to IE6 for enterprisey reasons, and until certain vendors upgrade and allow IE7+, then 6 is what we must use. However, I would not obsess over eye candy for IE6 users. I would encourage them to upgrade if possible, or else just tell them certain features cannot be supported. Especially if we're just talking about cosmetics.
That said, Curvy Corners is a JQuery plugin that looks for those proprietary CSS declarations, and will create image free curved corners in IE6. Be warned that there is a significant ammount of DOM manipulation going on, so it will slow down pages that have a lot of rounded corners.