Ionut Alex Chitu made a very good point last night about Greasemonkey and related add-ons. His post, entitled "Your Gmail Adds New Features", showcases three (last I checked) separate instances where people installed Greasemonkey scripts for Gmail and then forgot about them. In each case, the add-ons' features were interpreted as being part of the Gmail core, rather than add-on functionality available only in that customized browser.
Yesterday, the post I published about Garett Rogers' discovery of a tip-off to a supposed task list feature in Gmail was also revealed to be another forgotten add-on (RTM for Gmail, if you're wondering), a case I mentioned in the comments to Ionut's post. The title to Ionut's entry is quite apt, because it's your Gmail that gets the new features, and very few other users can confirm what you see. Google sometimes does have small test groups, but they're never (so far as I know) limited to one account…
So, what you can learn from this is to always remember what sites you have installed add-ons and enhancements for, and what scripts/extensions you have enabled. That way, when you see the new functionality in your browser, you won't rush to your blog and write a useless post about something that doesn't exist. (If you write fictional rumors, of course, that might be perfect inspiration, but you know what I mean.)
Actually, it just occurred to me that the people affected may have been using browsers on others' computers. In that case, ask the owner (or primary user) of the computer you're using if they've added any scripts/extensions/whatever that might be modifying the behavior of sites you're using.
Update (02/17): Garett insists in a comment at Google Operating System that he posted a screenshot of RTM accompanying his post about a bona fide task list feature in Gmail. The saga continues… And it looks like I was wrong, actually.