It seems the real reason Brosnan will not be returning to the role is (surprise)..... money. According to Variety the erstwhile James Bond was pushing for a compensation package that surpassed 40 million dollars in order to put on the black tux one more time.
Now I think that despite his Bond films being mediocre at best, Brosnan himself makes a great James Bond. I understand he was paid approximately $16 million for his role in Die Another Day, and I believe that the following quote which is being implied (by other movie news sites) to be in regards to the $40 million figure actually references what he was paid for the last film:
"...was an honest fee in terms of how much blood, sweat and tears I put into the role."
On his website he is quite gracious concerning the situation and doesn't seem like he's lost his marbles. I'll admit that his last Bond film did go on to earn over $430 million, and the main reason someone goes to see a Bond film is, well, Bond. But $40 million to play the role??