Apart from dust removal, cropping and basic adjustments like contrast / levels / sharpening, I don't do a lot of retouching / post-processing to my photos.
I am, unashamedly, a big fan of Steve McCurry's photography but I must admit it's disappointing that there's so much editing been going on - botched or not.
However, how much of this type of editing / post-processing is acceptable to us? For me, if the photo is intended to be a true representation of the scene viewed by the photographer, then I'd say very, very little to none. If, however, the intention is to produce a photo that's artistically pleasing, then that's where the lines get a bit blurred and liberties taken.
As has been said earlier, it gets even more blurred when the photographer is shooting to a brief and ultimate editorial control is their principal's (Nat Geo, etc).
As an aside - I shot a wedding many years ago (pre-digital) and, between the wedding day and production of the album, there'd been a serious family feud. I was (in all seriousness) asked if I could "remov"e one individual from the main group photos. I said, politely, no - mainly because I didn't know how to but also because it just didn't seem right to do so. I later heard that the bride's mother had tip-exed the offending individual's face off the photos I produced. Her call, I suppose, as she paid for the album...!
Anyway, I'm sure Steve McCurry is far from unique in having his photos "changed" - the others probably just have assistants who are better at digital photo manipulation.