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The message is mostly useful to alert people that it would be easier to check mobile XHTML syntax constraints with a markup validator if the doctype was a mobile-specific one (XHTML Basic 1.1 or XHML MP 1.2). That said, the mobileOK Checker does the markup validation against the XHTML Basic 1.1 doctype in any case and would report the errors there. The message should be flagged as an "informational warning", and not as a real warning.
See Ticket 133 in Unicorn trac: http://code.w3.org/unicorn/ticket/133