Here is a screenshot taken of the login form I see on my site using zpMobile.
http://steveluther.de/nologinbox.jpg
As you can see there is no login box so I can not type my username in.
This has been tested on 2 seperate android devices. 4.2.2 and 4.0.4 aswell as IOS 6.x and windows phone, the username login box is missing on all of them.