The only meaningful steampunk iPhone would be an intricate pocket novelty of gears and mechanical kinotropy that played music and movies on demand - and it still wouldn't be an iPhone, just an iPod.

It would be amusing to devise a rendering mask or texture that played movies as if represented by a simplified repertoire of moving shapes that might be provided by an analogue system of steam-driven gears: that would be the converse of a 'steampunk' iPhone, and rather fun.