AMEX holding out from being a part of Google Wallet’s latest upgrade could be interpreted as: a) no partnership being established between the two firms; or, b) AMEX is potentially planning on launching their own mobile payment solution; c) security concerns over the actual transaction processing itself; d) perhaps something else entirely.
Whatever the case may be, on balance, America finds itself woefully behind in yet another consumer adoption / implementation area that the nation pioneered (technologically speaking) which is (in a word) inexcusable.
(Source) CNET: http://goo.gl/D3iwJ