My "anywhere", "errands" and "phone" contexts are on my laptop, as well as my "someday/maybe" lists. That means if I don't have my laptop with me, I can't easily churn through my list of phone calls to make. If I do have my laptop with me, which is more likely, it's still a pain to pull it out, open it up and type in my password just to remind myself of whether I need to buy more Vegemite.
Possible solutions: paper-based system (yuck!) or some sort of portable electronic device.