I think it's interesting how many people blame Arsene Wenger in all of this, maybe cause I try to look on the big picture and gather as much information as possible. While I won't defend Wenger on all fronts, I think there are a lot of things most people seem to miss.
My theory is that the board in Arsenal doesn't want to spend a lot of money. They have said themselves that they want Arsenal to be a club that can reach success through young players without spending big cash on transfers, and the latest statement about this wasn't more then a few months ago. The big question that many people seem to ask is: "Why is Wenger still in Arsenal when he hasn't had any success the last years?" Interesting question, but the answer is not far fetched as people seem to think. Looking at what kind of money Wenger has gotten from the board, he has performed miracles. Under Wenger, Arsenal haven't missed Champions League a single season, and that is fantastic considering how little money they have spent, and if you compare that to clubs like Chelsea and Man City spending lots. The reason Wenger is still in Arsenal, is because the board knows that nobody could do a better job under these circumstances. If they would fire Wenger and take in a coach like Mourinho, he would say: "I need cash for player X and Y", and they would gently reply: "That's not how we do things around here".
Moving on, it's also a known fact that Arsenal were very close to signing Mata during the summer. "If Arsenal had the money, then what was the problem?" Again, when Arsene Wenger says that Arsenal has money to spend on transfers, I think he's putting it mildly. I think the truth is very different, and that even if the money exists, there is no way that the board would allow him to use it. The reason why Mata ended up in Chelsea was simple. Arsenal needed the cash from Fabregas in order to finance the move for Mata, but seeing as Barcelona were lazy and slow as always, Arsenal couldn't manage to complete the move in time, and Mata ended up in Chelsea. Wenger also said that he wanted to bring on a striker like Benzema, but again that is a player that would have costed quite a lot. Wenger is a reasonable trainer. He hasn't hesitated in offering the cash for the right players before, why would he do it now? Sure he has always had the whole "buy cheap & smart"-philosophy, but he has still put up cash for good players.
As some other people might know, David Dein (ex. Vice-Chairman) was a big key-figure within the club, and it's not strange that the club has been doing worse since his departure in '07. He had the ability to attract great players to the club, and he was certainly more of a "lets do this"-person, then the "whatever is fine"-people that runs the club now.
To sum things up: Wenger is easy to blame because he is the coach. I do think he has his issues, but that is more problems like how he handles the team during games. But people who say "lets fire him and get a manager who will buy players", they don't really seem to be seeing the REAL problem. Wenger is a smart coach and he has great experience, the Invincible-season is a big proof of that. Why wouldn't a trainer do anything if his team is sinking? The most reasonable explanation would be that his hands are tied.
/Just my five cents