There are a bunch of reports in the Google Webmaster Help forum where people are saying they are receiving emails that their Google accounts are "no longer verified by Google" to access their Webmaster Tools login.
The message reads:
[email address here] is no longer verified by Google. Please verify the address or select a different email address.
JohnMu from Google promised to look into the issue, he said:
This does seem fairly wide spread, although I personally did not have an issue.
Forum discussion at Google Webmaster Help.