With few exceptions, I’m not convinced there’s much value in using or migrating to xhtml at this point. In fact, if you’re not serving your content up as application/xhtml+xml or text/xml, there’s no reason at all to bother with xhtml and plenty of reasons why it’s a bad idea (http://hixie.ch/advocacy/xhtml). The fact that IE doesn’t even support xhtml is not the least of these problems. This isn’t just about closing your tags and quoting attributes: core javascript methods like document.write and properties like innerHTML can’t be used either. This alone makes switching to xhtml impossible for 99% of ad-driven sites.
The programmer in me loves the the idea of cranking out perfectly valid xhtml and writing javascript that only uses the DOM Core methods. For the same reason, I try to write html that’s semantic and avoid div soup and use CSS for presentation. But most people authoring web content are not programers and the low barrier for entry is partly why the web is what it is today. The fact is, html needs to be highly fault tolerant. I agree with those who have advocated valid markup for readability/maintainability purposes, but let’s be honest: we’re the exception, not the norm. Table-based layouts are only just starting to become obsolete (if there’s a bigger readability killer than that, I’d like to see it). And look at google’s web authoring stats: http://code.google.com/webstats/2005-12/classes.html. The 13th most popular class name is “mso:normal”. When that many people are using MS Word to author documents for the web, it becomes abundantly clear how far we have to go to migrate to xhtml.