[CGHMartini]

I got contexts for (1) various named Places, (2) lots of People, (3) "Think" and (4) "waiting for...". (1) and (2) are suitably hierarchical, of course.
I don't use "Phone" or "PC", as I've nearly always got a phone and my OR(MacBookAir, iPADs, Windows-NB) with me.... that context makes no sense to me - even though those contexts are examples in the tutorial on GTD.
If I need to call/email/kickass/kowtow Bob, that's context "Bob". Full stop.
Even when I'm waiting for something from Bob, that's still "Bob" - but with a start and due date. My context "waiting for..." is only for unimportant things that actually should happen automatically, e.g. getting a shipment from an online store after having completed the original action of buying something. I don't want to edit due dates there...

That having been said, I sometimes do get the urge to put actions in multiple contexts. I then split them up as a subgroup of actions.

One more thing: I try to broaden my Contexts and differentiate more on the Projects. Two reasons:
(a) I use Perspectives. Go to Projects view, select any amount of Folders/Projects you want, Focus, switch to Context, apply a primitive filter and save as a Perspective. Sync with iPad.
Doesn't work the other way round (you can't focus on contexts AND you can't sync projects-perspectives with iPad), so we need to differentiate on the only user-selectable feature, which is Projects.
(b) I use Places. For now that's iPad only - assign GPS-coordinates to Contexts and view your actions on an integrated Google-map. Nice. But gets extremely illegible if you have too many Places, so: broaden that definition, have less Places.