I tried flagging a few posts using the iPhone 4 / Mobile interface, and the layout far exceeds the screen size.
The initial box for flagging and closing looks good and survives a portrait to landscape switch well. The bug happens when choosing "exact duplicate" and the text input box exceeds the bounds of the screen.
This makes it very difficult to operate the buttons and provide a link to the dupe. My attempts to pinch / zoom or scale my way out of this failed. Rotating the device to landscape helped a little, but it was still not very workable.
Mobile Safari on iOS 4.3.5 - no jailbreaks, no betas.