A couple of main quotes:
Phishing schemes commonly involve e-mail requests for information that seem to be from trusted sources such as eBay or Citibank.
Why the scammers are after the usernames and passwords is, as yet, unclear. One possibility is to use the accounts to send spam. Another is the potential to search though the e-mail messages for any financial details left lying around in e-mails. With up to a gigabyte of storage per account, that's a lot of e-mail to trawl though.
You would presume that Google must of seen this coming along with all other kinds of abuse (spam my Gmail account)? At the moment their spam filters are not blocking the email, but they are trying to just warn everyone.