@Campbell Barton (campbellbarton), I'm just selecting the object and selecting "Weight Paint" in the popup menu, like you said, it works fine in 2.72, but @Sergey Sharybin (sergey) is right, in softwaregl mode it works, so it's something with my graphics card, (that is ancient, I know, but right now to change that is not an option, and it worked before anyways).