I'm going to set out my stall on this provocative question with a clear, decisive answer, and that answer is: it depends.
Here then, is my promise.
During combat, I might not be your friend. I want your PC's to be challenged; I want you to explore as many options as possible in combat; I want the situation to remain unpredictable and dangerous, even if you think you're down to the last foe. I want your enemies to make the most of their abilities -- and yes, that means blowing encounter and daily powers sooner rather than later -- as well as use whatever tactics are appropriate to their intelligence and goals. I will give the bad guys a similar level of situational awareness as you, and as much tactical thinking as I can get away with. I will give them a sense of self-preservation, and a dirth of outright stupidity. I will have them turn tail and collect reinforcements if all else is lost. I will have them dance on the bloodied remains of your shattered bodies, if that floats their boat. I will, in summary, play them as if they actually wanted to survive an encounter with a party of adventurers.
But fear not! I will not be cruel, or punitive, or spiteful. I will not coup-de-grace helpless PC's unless there is a very good reason for doing so... and said reason will be rendered moot if there are still viable, dangerous members of your party still to be dealt with. I will not try and catch you out with Gotcha! monster abilities, although I will take advantage of the sheer newness of 4E monsters and lay a surprise or two on you if I can. I will not manufacture situations that couldn't reasonably happen within the rules, although I may bend the rules on occasion for drama. These occasions will be overwhelmingly in your favour, but sometimes... very rarely... not. I will be as consistent in my rulings as possible, and when I fail in this duty, rest assured it will always be for the good of the game... not for the good of my own ego.
My goal is not to kill you. My goal is to challenge you, and for all of us to have a good time doing it. I'm at the table too.
Out of combat, I'll be nice. I will not hide clues from you or require you to make credibility-stretching leaps of logic to get from one encounter to the next. I will say, "Yes!" much more often that I will say, "No!", but only if the game is served by doing so. I will account for all of your characters and give them as much screen time as I can. I'll reward energy and imaginative thinking at the table even if I didn't account for it when I wrote or prepared the adventure... which will be most of the time, no doubt. I'll put life into the campaign and as much work as necessary to bend adventures around to your way of thinking. In short, I promise to make the game as fun and invigorating as possible, using the time I have available for preparation, and our precious hours around the table, to their best potential.
Claiming that there can be no adversarial relationship between the DM and the players is, if you ask me, nonsense, and yet many commentators seem to think this is the case. In any situation where there is a potential for failure, it is the DM's default responsibility to execute the challenge to the best of his ability. This will most likely be a combat, but it could as well be a skill challenge, a tricky bit of diplomatic roleplaying, or a trap. Whatever the situation, you do your players a disservice if you don't use all of the options at your disposal, and use them well.
This means preparing the encounter, learning about the abilities of a monster, and sparing a few minutes of thought for group tactics and the best execution of these abilities. 4E, more than 3.5, requires this of the DM because monsters are so varied now, with so many exceptional attacks that can change the nature of a battle quite significantly. If the players are steam-rollering their way through encounter after encounter, they will get bored, even if some of them won't admit it. One thing is guaranteed, the DM will get bored, and frustrated, and more and more inclined towards unhelpful and even spiteful behaviour in pursuit of the simple task of getting to play an encounter to its potential. Sometimes the DM might have to design encounters that accentuate the weaknesses of a party, and the players may balk at this, feeling unduly punished for maximising their characters. This is fine. Ask them if they'd rather just level their characters to 30th now and cut out the middle man, and see what they say.
The DM-player relationship, like any relationship, is a complicated one defined by context; the bottom line is that in D&D, rewards without challenges are dull for everyone involved. If they weren't, the game would consist of time-trials to see who could build a 30th level character fastest, instead of the complicated dance of mutual story-building and butt-kicking that the game is renowned for.
The DM's job is to provide the challenge. In this battle of wits, it's 5 against 1, and he needs all the help he can get.