It's best to avoid being in the situation of opening up a closed project in the first place; just start the project in the open if you can. But if it's too late for that, and you find yourself opening up an existing project perhaps with active developers accustomed to working in a closed-source environment, there are certain common issues that tend to arise. You can save a lot of time and trouble if you are prepared for them.
Some of these issues are essentially mechanical, and for these, the section called “Be Open From Day One” can serve as a checklist. For example, if your code depends on proprietary libraries that are not part of the standard distribution of your target operating system(s), you will need to find open source replacements; if there is confidential content — e.g., unpublishable comments, passwords or site-specific configuration information that cannot easily be changed, confidential data belonging to third parties, etc — in the project's version control history, then you may have to release a "top-skim" version, that is, restart the version history afresh from the current version as of the moment you open source the code; and so on.
But there can be social and managerial issues too, and they are often more significant in the long run than the mere mechanical concerns. You need to make sure everyone on the development team understands that a big change is coming — and you need to understand how it's going to feel from their point of view.
Try to imagine how the situation looks to them: formerly, all code and design decisions were made with a group of other programmers who knew the software more or less equally well, who all received the same pressures from the same management, and who all know each others' strengths and weaknesses. Now you're asking them to expose their code to the scrutiny of random strangers, who will form judgements based only on the code, with no awareness of what business pressures may have forced certain decisions. These strangers will ask lots of questions, questions that jolt the existing developers into realizing that the documentation they worked so hard on is still inadequate (this is inevitable). To top it all off, the newcomers are unknown, faceless entities. If one of your developers already feels insecure about his skills, imagine how that will be exacerbated when newcomers point out flaws in code he wrote, and worse, do so in front of his colleagues. Unless you have a team of perfect coders, this is unavoidable — in fact, it will probably happen to all of them at first. This is not because they're bad programmers; it's just that any program above a certain size has bugs, and peer review will spot some of those bugs (see the section called “Practice Conspicuous Code Review”). At the same time, the newcomers themselves won't be subject to much peer review at first, since they can't contribute code until they're more familiar with the project. To your developers, it may feel like all the criticism is incoming, never outgoing. Thus, there is the danger of a siege mentality taking hold among the old hands.
The best way to prevent this is to warn everyone about what's coming, explain it, tell them that the initial discomfort is perfectly normal, and reassure them that it's going to get better. Some of these warnings should take place privately, before the project is opened. But you may also find it helpful to remind people on the public lists that this is a new way of development for the project, and that it will take some time to adjust. The very best thing you can do is lead by example. If you don't see your developers answering enough newbie questions, then just telling them to answer more isn't going to help. They may not have a good sense of what warrants a response and what doesn't yet, or it could be that they don't have a feel for how to prioritize coding work against the new burden of external communications. The way to get them to participate is to participate yourself. Be on the public mailing lists, and make sure to answer some questions there. When you don't have the expertise to field a question, then visibly hand it off to a developer who does — and watch to make sure she follows up with an answer, or at least a response. It will naturally be tempting for the longtime developers to lapse into private discussions, since that's what they're used to. Make sure you're subscribed to the internal mailing lists on which this might happen, so you can ask that such discussions be moved to the public lists right away.
If you expect the newly-public project to start involving developers who are not paid directly for their work — and there are usually at least a few such developers on most successful open source projects — see Chapter 5, Organizations and Money: Businesses, Non-Profits, and Governments for discussion of how to mix paid and unpaid developers successfully.