The Gmail 5 million password hack that (probably) wasn't

Reports started circulating yesterday that Gmail had been hacked, with some 5 million logins at risk. This follows the publication, on Tuesday, of a plain text list of Gmail usernames and passwords on a Russian Bitcoin forum. Within 24 hours the 'hack hysteria' had taken hold and people were being advised to check if their accounts had been compromised, change their passwords etc. Trouble is, there appears to be absolutely no actual evidence that Gmail has been hacked at all, and plenty to suggest that this credentials list is just another composite; constructed with passwords taken from lists already published concerning other breaches. The Gmail connection is, at the most, that people whose credentials were exposed at those other sites and services had used a Gmail address to register their accounts.