It looks like Nancy Drew: Ghost of Thornton Hall is coming to the iPad! Her Interactive has a webpage for it and everything.
I'm guessing it's on the iPad, not the iPhone, due to the large screen resolution. They'd have to shrink the game window down to fit on the iPhone, and sometimes that causes problems. For example, if there's a small area that you need to tap on, it can be hard-to-impossible to do on a small screen.
The screenshots and character descriptions are the same as the PC version of the game, so I imagine that this is a direct port of the game, without much being changed. Wouldn't it be crazy if they did change a lot, though? Say the game has a new culprit. That'd be wild.