We are currently rolling out default https for everyone. If you've previously set your own https preference from Gmail Settings, nothing will change for your account. If you trust the security of your network and don't want default https turned on for performance reasons, you can turn it off at any time by choosing "Don't always use https" from the Settings menu. Gmail will still always encrypt the login page to protect your password. Google Apps users whose admins have not already defaulted their entire domains to https will have the same option.
To read about other steps you can take to protect your accounts and your computers, visit google.com/help/security.
Note: If you use offline Gmail over http currently, the switch to https is likely to cause some problems. Learn more about this known issue and how to work around it.
Original Source : http://gmailblog.blogspot.com/2010/01/default-https-access-for-gmail.html