Starbucks announced today that it is expanding its mobile payments system to Android users in Canada and the U.K., and integrating support for PayPal. That could be a big boost for PayPal’s mobile agenda considering that Starbucks now accepts mobile payments at 14,000 locations worldwide and its program has been remarkably successful while so many others have struggled. But as I noted earlier this year, that success isn’t much of an indicator of the potential overall market for mobile payments for several important reasons. The mobile wallet may eventually become a very real thing, but I don’t see any other retailer replicating Starbucks’ success any time soon.