I was going to go into all sorts of detail outlining a security hole in Gmail login and how hackers can use it, but I'd basically be re-hashing what others have already said. If you really need to know the how and why I'd recommend reading these two posts:
Login to Gmail and click on "Settings" in the top right corner:
In the settings window (General tab) scroll all the way down and click the circle next to "Always use https" and then click "Save Changes"
Then you never have to worry about it again. As an added bonus, I noticed that when I logged into Blogger to write this post it also gave me a secure login by default. Now I have to see if there's an easy way to make this the default in Google Apps Education Edition.