A photo of a mountain scenery published only for art's sake.
A magazine cover of a woman with a few scrapes from a car accident photoshopped out.
A makeup ad with a woman's wrinkles and blemishes photoshopped out, instead of adequetly concealed by their product.
A diet product with before and after photos doctored to show the customers thinner.
A hotel ad with an ocean view, which is actually photoshopped. The actual view is 50% obscurred by a building.
Read these examples and consider your feeling of discomfort with each. I think there is a sliding scale of 'wrongness' when using Photoshop and IMHO if a photo is used for an ad, it should have a disclaimer if any digital manipulation was used.