In a September 29, 2010 Googlewatch post, Clint Boulton explains that now Gmail users can turn off threaded view of their Gmail. Boulton said, "Hewing to its pledge to provide choice for its 180 million or so e-mail users, Google has backed off its adherence to its beloved "conversation view" by allowing users to receive e-mail messages in chronological order."
In another post TothePC offers detailed information on how to turn off the feature:
You can easily turn off conversation view in Gmail settings and get each reply listed as separate message in the inbox. [Read Detailed Easy Instructions]
Boulton said, "The thread, which Google calls conversation view, makes it super easy for users to find entire message exchange sessions between users. Ideally, this saves time. Still, enough people used Outlook, or Eudora for enough years to become comfortable with the traditional chronological view."