Looks like the rumor that originally surfaced in early June has come true.
Gmail has officially announced an option to turn off threaded replies. Those of you who don’t like "conversation view" (and judging from the previous post, there are at least a few of you out there) will soon be able to view chronologically-ordered Gmail messages one by one in your inbox.
The feature is being rolled out "over the next few days," so if you don’t have access to it yet, your wait is almost over. You’ll find a "Conversation View" section under the Gmail settings link when it’s available to you.
More on Techland:
Gmail May Offer Option to View Non-Threaded Replies