The Commons app for Android offers a login screen at start but it is quite customary in these situations to offer a possibility for new users to register.
PS: also related, it is not clear whether a user of e.g; English Wikipedia that has never logged in to Commons can use her Wikipedia credentials to log in or if her account needs to be enabled first in Commons as global account.
Version: unspecified
Severity: enhancement
Platform: Smartphone
URL: http://commons.wikimedia.org/wiki/File:Create-account-flows.png