One tweet from RIM says that there will be a "minor hardware bump" made to the Dev Alpha model in what it calls a "normal part of any pre-release
program." That was followed up with a question about cosmetic changes on the modified version of the BlackBerry 10 Dev Alpha phone. The tweeted response from RIM was pretty specific and stated that the microSIM placement had changed.
So we should be expecting a BlackBerry 10 Dev Alpha B device as RIM continues to court developers for its new BlackBerry 10 OS.