there are options for the configration of corners, shadow etc. for the most of the widgets. this option is missing for the textinput in jqm 1.3.0 beta and jqm 1.3.0 rc.1
I agree that we should add this.
Here is the custom CSS that you can add to have no corners or shadow: http://jsbin.com/erezak/7/edit
Slider (slider/flip toggle) and rangeslider (dual handle) are two other widget where option corners is missing.
Textinput: Added option corners. Fixes #5610.
Also removed class ui-btn-shadow because we don't use that.