[Toadling]

I imagine a lot of people store such information in a separate OmniOutliner file or text file, maybe linked to the related project. But I tend to do exactly as you describe: I log stuff, usually with a timestamp, in the notes field of my actions. If the piece of information is crucial for the next action, I put it in the notes field of the next action, otherwise it usually goes with the action just completed.

This has become an integral part of my work flow, and I frequently refer back to my old notes to refresh my memory on a particular outcome, what someone said, what I did or how I handled something, etc. I've been very happy with the way OmniFocus handles it, particularly because notes can be styled (great for making bold headings or embedding snippets of source code in a fixed-width font).

The fact that an action gets hidden after being completed isn't a problem for me since I keep the View Bar displayed most of the time and switching from Remaining actions to All actions is but a click of the mouse away.