I've never hit a CSS rounded corners solution that met all my requirements:

* No image files (Extra HTTP requests/inflexible to colour changes)

* No extra markup (All the benefits of a clean DOM)

* Cross-browser without relying on parsing bugs (Supporting user choice is good)

* No pixel-based measurements (Visual designs should scale to different browser text sizes and screen resolutions)

Given that, any solution will involve some trade-offs. To me, some non-semantic HTML is a reasonable sacrifice since it is transparent to most users.