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I love the W3.
Perhaps quoting from their short but sweet summary of quirks vs. standards mode will save someone else some research time (emphasis mine):
XHTML 1.0 can be served as HTML or XML. If you serve it as XML, use the MIME type application/xhtml+xml.
It is generally a good idea to use a DOCTYPE declaration at the top of an HTML or XHTML file so that the document is rendered in standards mode by more recent user agents.
The presence of an XML declaration in an XHTML 1.0 file served as HTML will cause your file to be rendered in quirks mode on Internet Explorer (and therefore for a potentially large proportion of your audience).
Insert your own rant about MSIE here.
Here are some other good pages on the subject for those of us for whom a short summary will not suffice:
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