It occurred to me today that there is no reason to assume that all of my lists need to be in the same place at all times. For example, if I'm out and about, I don't need to have my projects or someday/maybe list with me, but I do need to have my @Errands. If I'm at my computer, I don't need to have my @Errands list handy, but I'd better have my @computer list.
This means that I can maintain my next actions in media and formats that are appropriate to the context. I can keep my @Errands list in my Ubiquitous Capture Device (a moleskine notebook) and keep my @computer list in Outlook and not feel like I'm violating the integrity of my GTD implementation.
Boy, does this make sense now (after 4 years of using and abusing GTD)