Paul Starobin has a great piece about financial journalists who take speaking fees from financial firms, and the various policies that different media outlets have (or don't have) and enforce (or don't enforce).
It seems to me that for better or for worse, this problem is going to get worse in the future. As Paul Krugman wrote in 1996, one of the emerging properties of the digital economy is a structural shift away from selling output to selling appearances. We see this already most clearly in the music industry, where recording sales are stagnating but concern ticket revenues are skyrocketing. Lots of journalistic enterprises, including Slate, now stage various forms of "live shows" that we sell tickets to. And as individual writers more and more become personal brands, doing the publishing work will look more and more like a loss-leader for speaking. George Will is listed as demanding over $40,000 for a speaking appearance. Obviously a publication looking to cut off that sideline would need to pay Will a substantial premium over his current wage and my guess is that over time more and more publications will discover that their readers don't care enough about the potentially corrupting influence of speaking fees to make it worth paying that premium. And yet the corrupting influence seems clear and real. Not in the sense of a bribe ("write nice things and we'll hire you as a speaker") but in the sense of Lawrence Lessig's dependence corruption, where prominent media figures come to count on income streams that require them to be well-regarded by the sorts of people who are in a position to pay them to speak.