I think the legalities lie in whether you say something like "An official Brad Pitt Puppet" vs. Brad "Bitt" or something like that. Also, if you have a spin on it all your own and they're a public figure, you should be in the clear.
There's also a difference when you do a one-off of a celebrity vs. doing a mass market selling to get them into stores. You may also want to shy away from casting the celebs in a bad light (advertising a Michael Jackson puppet as a creepy guy that hunts down children is probably a no-no).
Hope this helps.