For the short version, read Rob Graham at Errata Security.
Google detected someone attempting a man in the middle attack using a certificate issued in Turkey. TURKTRUST issued two subsidiary Certificate Authority certs which allowed whoever had them to sign any certificate they wanted, for any domain they wanted. Yes, this is how SSL works and it’s a big mess (I talked about it a little in 2011).
Google likely detected this using DNS Pinning. Every version of Chrome checks any Google certificate against a list of legitimate Google certificates, which they build into Chrome itself. If there’s a mismatch, Chrome detects and can report it.
Nice, eh? That’s why Rob says don’t mess with Google. You try to MitM any of their domains, and if any users run Chrome they are likely to find out. Everyone else (who can) should do this.