It was about 6 months ago that I stopped making images for my rounded corners, and instead started using CSS3 to create them. I always allowed my designs to "work" without the rounded corners, they just didn't feel as nice. This allowed IE8 and below compatibility.
I recently started experimenting with PIE (http://css3pie.com) and was pleasantly surprised. simply add to your css file:
behavior: url('PIE.htc');
and several CSS3 features, including rounded corners, now work in IE6-8.