Hmmm, replacing UICamera.cs doesn't seem to fix the issue I am having.
If I open an Amazon IAP dialog (native small pop-up that overlays on top of the game, covering about 1/3 of the screen) on the Kindle, cancel it, and open it again, cancel, open...etc after about 20 iterations, the UI is no longer taking any input. It's not frozen, I can see stuff animating and the system back button properly navigates backwards, its just that no UI is responsive.
(FWIW, the game is 100% UI done via NGUI)