The proof is piling up that Twitter will add a "delete tweet" feature, but that those who want it will have to pay for it: app researcher Jane Manchun Wong discovered a subscription screen linked to the feature. The ability to undo tweets was also listed in a survey asking Twitter users what features they'd pay for, as well as in the app's code, and CNET received confirmation from Twitter today that it is being tested.
It's uncertain when the feature will be widely available, but it's understandable why Twitter would reserve it for its future subscription service: what better lure than a "keep yourself out of trouble" button?
Twitter, for one, hasn't said if the functionality would be restricted to paying users — it's likely the decision hasn't been taken yet because the feature isn't far enough along in growth.
It's also worth mentioning that the button's nature indicates it functions in the same way as Gmail's "delete send" button, which simply waits a few seconds before sending the message. In other words, it may not be the edit button that people crave, but it can bring you close (albeit without the part where anyone else will tell you that you've made a mistake).
If Twitter is open to suggestions for the undo button, here's one: make it show if anyone sends an offensive tweet without first reading the warning, allowing them a few more seconds to rethink. It's difficult to tell if this will boost Twitter's overall environment, but it's worth a shot, right?