edit: wow, this turned into quite a rambling post - sorry folks!
Lying is wrong. That's what my mum always told me. Well, actually it might have been Sesame Street, but it's still true.
Company representatives really ought to learn that today, with everything having access to this interweb thingy, their words will not be quietly forgotten after getting everyone hyped up, but rather will be recorded and dissected and challenged - they will come back to haunt them. So they ought to be a lot more careful to only actually say things that can be backed up later.
That aside, much of what is said probably isn't a lie, per-se. During the initial E3 launch of PS3 for example, they were careful not to state at the time that anything was real (except the bits that definitely were). So while some people might get that impression, technically they haven't lied. Whether they have mislead or not is another matter - I tend to think that people really need to take a bit more personal responsibility when it comes to advertising (which is effectively what any launch or press-conference really boils down to). If it's not overtly stated that something is realtime, assume it isn't. But you can't claim it's a lie either.
After the fact, you do get less well briefed members of the company making stupid comments. For example I've seen people from Sony, subsequence to the slick launch, come out and say things were real that (pretty clearly) were not. I really don't know what was going on in their heads at the time, and it's entirely possible they knew this and were lying, but I would also think it entirely possible that they simply don't know, or themselves misinterpreted the information they were given. Or they're just not used to being spokesmen and aren't thinking before they speak.
I think this latter type of statement is what needs to be cleared up - companies internally need to give much clearer briefs to all staff on exactly what can and can't be said.
Meanwhile, as a developer, I've been on the other side of the fence. I've seen people outright lie about a render actually being a screenshot, and so I know it happens. But I've also seen rampant incompetence where people will simply just ramble on about something they actually know nothing about, but where they clearly actually believe that they do.
These people aren't liars - they're just idiots - but both probably need to shut up.
So would I doctor a screenshot, or be a party to that kind of thing? Yes - yes, I would. Only, I might add, if I was confident that the result could and would ultimately be achieved - but otherwise, yes.
Sometimes the results will fall short of that, which I suppose in some eyes will make me a liar. Other times, we'll exceed it - am I also a liar there? Or is the lie acceptable if I've underestimated?
I'd be happy to say to people "this isn't a real screenshot, but actually a mockup of what we think will be achieved" (though you'd have to make it a bit more concise than that I suppose). Paradoxically however, I think you'd get even *more* people calling you a liar, if you were open about it not being real. So I'd probably be just as happy to play it down and simply say "here's what the game will look like", provided we didn't go too far and say "this is a 100% realtime shot from the engine".
Let me just tell you why I'd be willing to do this though...
A while back I worked on a game where early on in development, we were asked to explain the title to the marketing people so they could put together a campaign for it. We provided them with some material that explained the various aspects of the game, combined with some screenshots - taken directly from the game - to show everything in action. These screens were pretty dreadful - nothing was finished, so half the art was missing, stuff wasn't textured, lots of place-holder graphics and stuff... generally bad. But you could see how the game would be played, which was kind of the point.
Except the marketing department promptly released the screens to the press. We were slated, the game disappeared off everyone's radar, and we never really recovered. We never got any more marketing because they'd written it off so despite good reviews, it bombed. The screens were certainly real - but at anything but a near-final stage of a game, a screenshot is quite often going to look pretty poor compared to the finished product. Certainly in our case the finished game looked vastly better than the early screenshots - I'd have to say that I'm quite proud of what we achieved - but no-one really noticed. Complete honesty served no-one.
So in that same situation again, I'd happily provide doctored shots instead. And then I'll work my arse off trying to make the game look as good or better.