"Have you ever used Google Search on your smartphone and clicked on a promising-looking result, only to end up on the mobile site’s homepage, with no idea why the page you were hoping to see vanished?"

[googlewebmastercentral.blogspot.sg...]

Such pages now get a line with "May open the site's homepage" and a link "Try anyway" in the SERPs.

Additionally, webmasters of such sites are informed in G's Webmaster Tools.