A Google Webmaster Help thread had one webmaster who received an email from Google warning of a phishing attempt on this person's site. The webmaster asked if it was real. The email said he/she should "submit a safebrowsing report at http://sb.google.com/safebrowsing/report_error/."
JohnMu from Google replied that if you want to make sure the email is real, that you should login to the Google Webmaster Tools and see if there is a message there about it. If yes, then it is legit, if not - not.
In addition, John explained that "the sb.google.com URL you mentioned is the correct one to submit feedback on the safebrowsing status of the flagged URL on your site."
Give sb.google.com a try. :)
Forum discussion at Google Webmaster Help.