Guideline 6. Ensure that pages featuring new technologies transform gracefully.

It seems it would be kind of tough to keep your site on the "cutting edge" but still have it be compatible with older browsers, but that is exactly what this guideline is about. If it comes down to choosing to put in some fancy new bit of code, or making the site easily accessible for all, then this guideline should clear up any confusion about how the situation should be handled.

6.1 Organize documents so they may be read without style sheets. For example, when an HTML document is rendered without associated style sheets, it must still be possible to read the document. When content is organized logically, it will be rendered in a meaningful order when style sheets are turned off or not supported.
6.2 Ensure that equivalents for dynamic content are updated when the dynamic content changes.
6.3 Ensure that pages are usable when scripts, applets, or other programmatic objects are turned off or not supported. If this is not possible, provide equivalent information on an alternative accessible page.
6.4 For scripts and applets, ensure that event handlers are input device-independent.
6.5 Ensure that dynamic content is accessible or provide an alternative presentation or page.

Some applications or examples of this would be when working with Java or Flash.

Source: http://www.w3.org/TR/WCAG10/#gl-new-technologies