So I tried Tiffen's Photo fx Ultra app with impressive results. The photo I used was taken using my iPhone 4S in Cabo San Lucas last February. The story is that this beach was the scene of the original Planet of the Apes when (spoiler alert!) Charlton Heston discovers the Statue of Liberty. I've heard conflicting reports, but it's a nice story, a nice tourist attraction, and an even nicer beach.
I used the app's film grain feature to make it look like the image came from a 1970's era film reel. I also experimented with lighting options. It seems every feature has a dozen or so settings to fine tune the results.
The regular app works on an iPhone but the Ultra version requires the iPad as the added real estate of the iPad makes it more powerful. You might be able to do away with Photoshop after all. The ability to touch the screen and manipulate the image directly feels so much more powerful than using a mouse.
Tiffen is known for their high quality photography equipment and they have clearly brought their expertise to the app world.