Yesterday, Google sent out about 20,000 warnings to webmasters that have had their sites compromised and may have no idea about it.
Matt Cutts tweeted they sent out these notifications of "your site might be hacked" for sites suffering from "weird redirects."
Recently, I have seen several sites reported to have weird redirects but the redirects are hard to track down. The hack is where the hacker gains access to your HTACCESS and only redirects users who click from Google to your web site. Otherwise, if they type in the domain name directly, there will be no redirect.
Here is Matt's tweet:
Is your site doing weird redirects? We just sent a "your site might be hacked" msg to 20K sites, e.g. goo.gl/S6Ptk— Matt Cutts (@mattcutts) April 16, 2012
Forum discussion at Twitter.