The company wrote in a blog post, "Using https helps protect data from being snooped by third parties, such as in public wifi hotspots. We initially left the choice of using it up to you because there's a downside: https can make your mail slower since encrypted data doesn't travel across the web as quickly as unencrypted data. Over the last few months, we've been researching the security/latency tradeoff and decided that turning https on for everyone was the right thing to do."
Google is in the process of rolling this out to all users. In good 'ole Google fashion, the company is still giving its customers choice. If you want to turn https off, you can do so by making the change in the settings menus.
For businesses using Google Apps, users of domains not already encrypted (by admins) can still use http Gmail. If site admins have set up https, however, the option to turn https off won't be available.
Google cautions that offline Gmail users will likely encounter problems once the switch to https is complete. Google has a few suggestions for those users here.