Just wanted to bump this thread with some additional information...
I'm seeing this same problem in my game on both my Nexus 7 and Galaxy S3 (though, not with some older phones I've tested on). The interesting thing, however, is that I've only started seeing this problem after updating to Unity 4.1.3 and NGUI 2.6.1. I was previously using Unity 3.5.7 and NGUI 2.3.something.
Prior to updating these things, I had a proper (i.e. white background) input field box in which a user could see what he/she was typing across all devices that I've tested on. Now, I have a similar transparent mess as seen in the earlier screen shot. I can't imagine how the NGUI upgrade has anything to do with it, which makes me think that perhaps something in Unity 4.x.x has changed things.
I use the Swiftkey keyboard on my phone/tablet so, just for good measure, I also tried toggled back to using the stock keyboard. I was greeted with the same transparent input field box.