In a quiet announcement on their Docs blog, Google said they would not be supporting Firefox 3.5, Internet Explorer 7, and Safari 3 or older browsers as of August 1.
They warn "in these older browsers you may have trouble using certain features in Gmail, Google Calendar, Google Talk, Google Docs and Google Sites, and eventually these apps may stop working entirely."
Obviously Google programs will work best in their browser, Chrome, which conveniently is top of the alphabetic list for updating browsers they include in their announcement. The push to gain more of the browser market is coming from many of Google's products. They have occasionally promoted it on the search page, and is branding it well on its first television ads.
The push for updating browser is also to take advantage of HTML5 which has just recently been interacting with browsers and having problems in older versions. As more interactive capabilities are introduced to the web it becomes important to have the latest version of browsers that support the technology.
Latest stats on Chrome put it between 12.52 and 25.6% of the browser user market. If their programs cease working in some browsers they could be getting antitrust problems along with an increase in market share.