Really. See how it validates a page built on my Scherzo WordPress theme.

On the face of it, this isn’t a bad result. Two low importance failures and one medium.

It’s just that the majority of the problems have nothing to do with how well the page performs on a mobile device:

• it generates a slew of errors because I don’t use XHTML. I don’t use XHTML because XHTML has been dead for about a year.
• it doesn’t like the i tag I use. Apart from the fact that i has been gloriously reinterpreted, I don’t see how its usage has ever affected how a mobile device renders the page
• it frowns upon the @media queries I use, even though they contain styles for modern browsers. If your device doesn’t understand @media you get the default, mobile friendly stylesheet

In short, it’s pretty useless. I assume it hasn’t had any love for a while.