HTML 5 is still a long way from becoming a standard...years in fact. That doesn't mean that you can't use it now. Most of the web browsers out there already support HTML 5, including Microsoft Internet Explorer 9 beta. You don't need to flush your current XHTML website--XHTML is a finished specification, and will remain so for the future.
As far as HTML 5 being a plug-in and Flash killer, that remains to be seen. "Just as when CSS Web fonts weren't widely supported and Flash was used in sIFR to fill the gaps, Flash also saves the day by making HTML5 video backwards-compatible. Because HTML5 is designed to be 'fake-able' in older browsers, the mark-up between the video tags is ignored by browsers that understand HTML5 and is rendered by older browsers. Therefore, embedding fall-back video with Flash is possible using the old-school <object> or <embed> tags, as pioneered by Kroc Camen is his article 'Video for Everybody!'."