The current guideline is W3C Accessibility Guidelines 1.0.
There are many features in HTML 4.01 and XHTML which are now deprecated (depending on the version–strict, transitional or loose). A deprecated element or attribute is one that has been outdated by newer constructs.
Authors should avoid using deprecated elements and attributes. User agents should continue to support them for reasons of backward compatibility.
The next version of the W3C Accessibility Guidelines will be 2.0. These are expected to become an official recommendation in late 2006.
You can view the latest Working Draft and also the mapping of WCAG1.0 checkpoints to WCAG 2.0 success criteria.