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IE6 does not support the mime-type "application/xhtml+xml", so IE6 does not support true XHTML documents (as Robin states above). However, IE will display XHTML documents that are still being served as "text/html" (XHTML in HTML compatibility mode), so your pages are safe.
So, if you are really taking advantage of the X in XHTML then you need to serve it as "application/xhtml+xml" to those browsers that support it (Firefox etc.) and as "text/html" to those that don't (IE6...). All sounds a bit messy!
Most people say they are using XHTML, but what they are really doing is serving XHTML as good old fashioned HTML.
Does IE7 support this mime-type?
HTML4.01 isn't going anywhere anytime soon. If you always close all your tags, use lowercase tags (because XML is case sensitive), no empty tags etc. etc. then you should be pretty safe if you do have to switch.