Count me in on this problem, although, like many, I found one specific plugin that did fix the issue. Here’s what I tried:
a) Clearing browser cache(s)
b) Switching to default theme
c) Replacing widgets.php with one from the the installation .ZIP
All failed.
It wasn’t until I selected all of my available plugins and deactivated them that the button returned. When I re-activated all of the same set, the button disappeared again. That meant it was something specific.
By process of elimination, the problem plugin was EXEC PHP 4.9. With about 30 other plugins installed – no problem. As soon as I add EXEC PHP, the button disappears.
I’d be interested if any of you here even are running EXEC PHP. Another user in a thread said it was their flickr plugin, but I didnt even use that myself.
Hopefully they can fix this bug in 2.72!