Straight from the horse’s mouth, Adobe themselves have guidelines on their website stating how Photoshop, being a trademark should never be used as a verb:
The Photoshop trademark must never be used as a common verb or as a noun. The Photoshop trademark should always be capitalized and should never be used in possessive form, or as a slang term. It should be used as an adjective to describe the product, and should never be used in abbreviated form. The following examples illustrate these rules:
Trademarks are not verbs.
CORRECT: The image was enhanced using Adobe® Photoshop® software.
INCORRECT: The image was photoshopped.
I’ve been saying this for years, and in fact this was the title of an essay I wrote back when I was in grad school. This is all part of the comoditisation of design, and the fact that design services and tools are being packaged and bundled like free stickers and glittery string. I really should refine that old essay and make it more relevant (and therefore less academic). [link coming soon]
Of course Adobe found themselves with egg on their face because some geek turned up the verb itself on a section of Adobe’s own site!:
The broken egg photo in the big poster was retouched, since the broken egg photo was taken independently and then photoshopped over the invitation form.