Hi,
i can confirm this focus bug. I made an empty project with only two Input fields and when I start the program on WP8 I can focus on my first Input field but only once. After first focus and defocus it breaks and will only focus for a time of one frame.
EDIT: I'm using Unity 4.5.0f6 and NGUI 3.6.4
EDIT2:
I added two gui elements to watch TouchScreenKeyboard.hideInput and TouchScreenKeyboard.visible fields' values and it seems that after I touch UIInput-field visible-value goes to true for one frame and then false for one frame and after that it stays on true value untill I close the touchkeyboard.
Tomi
Skypetm: tomikokkonen