Also, while I don't mind running a game where players assert narrative control (e.g. Burning Wheel), most of the group prefers that we not run D&D that way. So if the player asks if there is a mage guild in town, the decision I make is solely on the nature of the campaign, not some "cool" idea that the player has.
We don't modify the campaign, on the fly, instantly, for some idea the player just got--because the players specifically do not want the world to be so mallable that it changes based on questons they ask.
Part of the fun of being a clever explorer is knowing which questions to ask, and you can't get that fun if the thing you are asking questions about is changing during the process.
Expanding on that, alot of the time when a player asks for or about something, the question that they ask is really standing for some other implicit question.
For example, if the player asks, "Is their a mage guild in town?", the real implicit question is usually, "Is their a wizard around I can talk to?" The answer to the question, "Is their a mage guild in town?" is almost certainly going to ultimately be, "No." But the answer to the question, "Is there a wizard around?" is almost certainly going to ultimately be, "Yes" So one of the ways I try to, 'Say "Yes"' is to try to answer the big question by addressing the actual need. Most of the time, you don't need a 'Wizard Guild', you just need a wizard.
Initially though both questions are going to be answered with one or both of the following:
a) Make a knowledge check.
b) You don't know, "Why don't you ask someone."
The single biggest mistake a new player can make at my table is trying to interact with me rather than my environment. I admit to not being very mallable. If you try to manipulate me, you'll get immediate resistance. But if you try to manipulate the game environment, I find that fun and will be much more accomodating. I'm perfectly willing to assume that its a big big world and there are lots of things in the gray areas that I haven't detailed but which can be filled in as needed. What I won't believe and am not willing to assume is that there is anything that the PC's may need or want or think that they need convienently located nearby all the time.
I should note that I wouldn't want to be a player in such a world either.
Likewise, while I may rely on the player's to help me fill in the details when they request things I haven't anticipated, I won't rely on them to fill in the big, obvious things that would stand out. The reason that there is almost certainly not a Wizard's Guild in the town is that such a big, imposing, exotic feature would have been something I designed the town around and would have been mentioned centrally already.
But while the average player who says, "Is there a Wizard's Guild in town?" means only, "I need to find a wizard. Don't they hang out in guilds?", occasionally you'll find players whose question is not so innocent. These are the players that are asking, if not demanding, that not only be some wizard of some sort be available but that incredibly powerful wizards be made available for them in large numbers. These are the same sort that will argue with you that because of X, said wizards ought to do Y for them and that you are bad DM for not seeing that. Indeed, if you don't make a vertible army of tame wizards available to them in the first place, then you are bad DM running an antogonistic and unrealistic campaign. How dare the DM say 'No' to such a reasonable request! You are ruining the players fun!
If you read the 1st edition D&D, Gygax pretty much tells you to treat every request or question of this nature as inherently hostile and adversarial. I think that goes to far, and fosters the worst sort of player vs. DM dynamic of sterotypical 1st edition D&D. But I think 4e goes too far the other way. It is not reasonable to assume that there must be any item you'd want in the wagon of every travelling peddler. It is not reasonable to assume that every town has a wizard guild just because it would be conveinent for the PC (or they think that it would). You don't have to say 'Yes' to everything, and you can easily ruin a game by saying 'Yes' to everything just as you can ruin one by saying 'No'.