Well, actually, if Google asserts the right to your message content (and they do) then the gov't would only need a warrant if Google declined to turn over your e-mail to them.
And I quote:
"11.1 You retain copyright and any other rights you already hold in Content which you submit, post or display on or through, the Services. By submitting, posting or displaying the content you give Google a perpetual, irrevocable, worldwide, royalty-free, and non-exclusive license to reproduce, adapt, modify, translate, publish, publicly perform, publicly display and distribute any Content which you submit, post or display on or through, the Services. This license is for the sole purpose of enabling Google to display, distribute and promote the Services and may be revoked for certain Services as defined in the Additional Terms of those Services.
11.2 You agree that this license includes a right for Google to make such Content available to other companies, organizations or individuals with whom Google has relationships for the provision of syndicated services, and to use such Content in connection with the provision of those services."
Google can choose to turn your e-mail over to the government, or the Chinese or the Scientologists or Lord John Whorfin if they wanted to. This ruling only says if they tell the gov't to sod off that the gov't has to come back with a warrant to compel them to change their tune.