Have been a few times I've started a new thread, submitted it, then realised I made a typo in the title. Having it editable woudl definitely be a useful thing. A few times I've seen people complain about someone's title (recall the one regarding undercover officers being among the mob of protestors last month) and a few people complained about the inappropriate title. It'd be nice if the one who posted it in the first place could edit as easily as the message.
BTW, a couple/few years ago, weren't we able to edit the thread title?