Why not competing commodity currencies?

Over the past week or so the blogger Megan McArdle has offered several critiques of the gold standard and particularly Ron Paul's case for it, see for example, here, here, here, here, here and here. In response Pete Boettke at The Austrian Economists has suggested that she should read what George Selgin and Larry White have written on the topic of competitive monetary regimes. Now Larry White himself has jumped in over at Division of Labour.