I've been playing around with a similar concept for the last year or so, with contexts like "On the go", "Have / Do not have internet connection", "Access to work intranet", etc., and whether a phone could do a good enough job of determining whether you're in a collection of those contexts.
Thanks for pointing me at OmniFocus, it looks really cool. Unfortunately, I'm an Android/Windows guy, and I doubt the Web version is going to be good enough at contexts for it to be worth it for me... :(
Or really, most any Note/ToDo that has first class support for GTD aka Getting Things Done which leverages the notion of @contexts.