Many people at my company, including me, are diehard users of OmniOutliner, and find outlining to be incredibly useful in many scenarios. Unfortunately cross-platform collaboration is difficult with OmniOutliner; in fact even single-platform collaboration isn't easy, since it's file-based.
We love Google Docs in general, because of its excellent collaboration abilities, but its outlining support is weak, annoying, and frustrating.
I hope there are some good options out there!