From Google’s blog:
We’ve worked hard over the past few years to increase our services’ use of an encryption protocol called SSL, as well as encouraging the industry to adopt stronger security standards. For example, we made SSL the default setting in Gmail in January 2010 and introduced an encrypted search service located at https://encrypted.google.com four months later. Other prominent web companies have also added SSL support in recent months.
As search becomes an increasingly customized experience, we recognize the growing importance of protecting the personalized search results we deliver. As a result, we’re enhancing our default search experience for signed-in users. Over the next few weeks, many of you will find yourselves redirected to https://www.google.com (note the extra “s”) when you’re signed in to your Google Account. This change encrypts your search queries and Google’s results page. This is especially important when you’re using an unsecured Internet connection, such as a WiFi hotspot in an Internet cafe. You can also navigate to https://www.google.com directly if you’re signed out or if you don’t have a Google Account.
Read the full blog entry on Google.