Currently, the plone.org web site itself is using XDV for its theme—so it's battle-tested and ready for serious, high-traffic sites.
Before starting, let us explain briefly the history of XDV, and the reason it exists.
When people talk about this new approach to theming, they will often refer to the general approach as Deliverance-based. The original Deliverance project (http://deliverance.openplans.org) was started by Paul Everitt (http://pauleveritt.wordpress.com) a long time ago, and was further enhanced by Ian Bicking (http://www.ianbicking.org), who is its current maintainer.
Along the way, Deliverance got more powerful and expanded beyond the initial goals, and started handling cases that were not included in the original scope.
Long story short, a new implementation of the same basic approach was started, called XDV. This is a stripped-down, pure XSLT implementation of the Deliverance concept, and can be compiled down and used...