Gmail now encrypts all the network traffic by default:
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.
Tip: Mailplane has its own security setting: Use a secure connection (https) under Preferences > Accounts. If turned on it'll open Gmail with the https protocol, even if the Gmail setting isn't set to use https. The Mailplane preference was introduced in 2007, before Gmail had the https setting. It is always turned on by default, too. There's a good chance you always used https in the past.
Important information for "Offline Gmail" users: If your can't sync anymore, it's because of the https settings change. Check out this help page for the solution.