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I don't think there's any official app for PBS for the Ipad. But what you can do is to go to PBS in Safari.
Then hit the "Share Document" button at the top left. (Looks like a piece of paper with an UP arrow on it.)
Then chose the "Add to Home Screen" button.
This will put a button on your iThing, on the desk top, which will take you straight to the web page. You can set this on Discussion, Search or what ever part of PBS you want to visit often.
I have mine set to Member's Thoughts, and it even has the PBS logo on the button. Like here, once in a while it'll as for your password.