Web developers make the mistake of trying to make their site look the same on all browsers and abandoning older browsers completely if they can't get it right. Just create a basic css file for older browsers that don't support css3, so that your site is still readable. Don't even attempt to port back features it doesn't have like border-radius.
If you're not a website-artist website whose sole purpose is to show off swooshy website templates, then your website's purpose is not to show the template. It is to show the content.
If you're turning people away because your template doesn't work on their browser, well, you're stupid and you've misunderstood the entire point of your own website.
Even IE-5 can do basic text rendering and forms, and frankly that's all you need to run most websites. If your site doesn't work on that browser - give them a basic site. Not only will people stuck with IE6 thank you for it (and not immediately go to your competitors), but people with disabilities such as partial blindness who hate funky website designs will be thanking you (and not suing you) too.