I'd like to mention Touch Punch, a js library that you can use to add drag-and-drop functionality to mobile devices.
If you are using jQuery and jQueryUI, just add Touch Punch and it works like a charm.
The URL to get it is: http://touchpunch.furf.com/
I used it here: http://whaddayaknowabout.com/criticalthinking/premiseandconclusion/