Starting with the first 1.8 snapshot and beyond, the "Minecraft Realms" button is gone from the main menu! I know I am an alpha tester, for I can see and still use the Realms feature in 1.7 and below. I don't know if it's a feature (the button doesn't show up if no realms you play on are up to date with the version you're using, or something), but just in case.