I generally use the first method you describe: keep a project outline with the project support materials. For many of my projects, my outline isn't formal; it's a piece or pieces of paper with an outline of all of the steps I think need to be accomplished, grouped or sorted either by the order they should be done in, the people that need to do them, the context they need to be done in, the tool (e.g. a computer) required to do them. In a perfect world, this is the result of the project-planning process described in GTD, and it follows a brainstorming session. I keep it in my support material, which is either in my desk at work or my desk at home, because I usually don't try to do other planning when I'm not either at my desk at work or at my desk at home. And if I have my NAs up to date, I usually don't have to refer to it very often during the day.
I use Outlook at work and a palm (native apps except for Wordsmith as the memo reader), and I use the Palm Desktop at home. I've toyed with keeping an entire project plan in the notes field of my project identifier, Shadow, Bonzai, and probably something else, and I found that keeping that system up to date was more trouble than it was worth: I was spending more time updating the system than doing things (not literally, but it felt like it). I'm more and more stripping down my GTD approach to the simplest form possible.