If you're a paid developer on a project, then set guidelines early on about what the money can and cannot buy. This does not mean you need to post twice a day to the mailing lists reiterating your noble and incorruptible nature. It merely means that you should be on the lookout for opportunities to defuse the tensions that could be created by money. You don't need to start out assuming that the tensions are there; you do need to demonstrate an awareness that they have the potential to arise.
A perfect example of this came up early in the Subversion project. Subversion was started in 2000 by CollabNet (http://www.collab.net/), which was the project's primary funder and paid the salaries of several developers (disclosure: including myself). Soon after the project began, we hired another developer, Mike Pilato, to join the effort. By then, coding had already started. Although Subversion was still very much in its early stages, it already had a development community with a set of basic ground rules.
Mike's arrival raised an interesting question. Subversion already had a policy about how a new developer gets commit access. First, she submits some patches to the development mailing list. After enough patches have gone by for the other committers to see that the new contributor knows what she's doing, someone proposes that she just commit directly (that proposal is private, as described in the section called “Committers”). Assuming the committers agree, one of them mails the new developer and offers her direct commit access to the project's repository.
CollabNet had hired Mike specifically to work on Subversion. Among those who already knew him, there was no doubt about his coding skills or his readiness to work on the project. Furthermore, the non-CollabNet developers had a very good relationship with the CollabNet employees, and most likely would not have objected if we'd just given Mike commit access the day he was hired. But we knew we'd be setting a precedent. If we granted Mike commit access by fiat, we'd be saying that CollabNet had the right to ignore project guidelines, simply because it was the primary funder. While the damage from this would not necessarily be immediately apparent, it would gradually result in the non-salaried developers feeling disenfranchised. Other people have to earn their commit access — CollabNet just buys it.
So Mike agreed to start out his employment at CollabNet like any other new developer, without commit access. He sent patches to the public mailing list, where they could be, and were, reviewed by everyone. We also said on the list that we were doing things this way deliberately, so there could be no missing the point. After a couple of weeks of solid activity by Mike, someone (I can't remember if it was a CollabNet developer or not) proposed him for commit access, and he was accepted, as we knew he would be.
That kind of consistency gets you a credibility that money could never buy. And credibility is a valuable currency to have in technical discussions: it's immunization against having one's motives questioned. In the heat of argument, people will sometimes look for non-technical ways to win the battle. The project's primary funder, because of its deep involvement and obvious concern over the directions the project takes, presents a wider target than most. By being scrupulous to observe all project guidelines right from the start, the funder makes itself the same size as everyone else.
The need for the funders to play by the same rules as everyone else means that the Benevolent Dictatorship governance model (see the section called “Benevolent Dictators”) is slightly harder to pull off in the presence of funding, particularly if the benevolent dictator works for the primary funder. Since a dictatorship has few rules, it is hard for the funder to prove that it's abiding by community standards, even when it is. It's certainly not impossible; it just requires a project leader who is able to see things from the point of view of the outside developers as well as that of the funder, and act accordingly. Even then, it's probably a good idea to have a proposal for non-dictatorial governance sitting in your back pocket, ready to be brought out if there start to be indications of widespread dissatisfaction in the community.
 See also Danese Cooper's blog post, preserved in the Internet Archive's Wayback Machine at https://web.archive.org/web/20050227033105/http://blogs.sun.com/roller/page/DaneseCooper/20040916, for a similar story about commit access. Cooper was then Sun Microsystem's "Open Source Diva" — I believe that was her official title — and in the blog entry, she describes how the Tomcat development community got Sun to hold its own developers to the same commit-access standards as the non-Sun developers.