Someday vs. Maybe
I've been using Things for Mac for about a year now, after using the GTD Add-in for outlook for several years. One of the features I like about Things is the ability to Schedule a project or task. This removes the project (or next action) from your next action/project list until a specific date.
This allows me to move projects (and next actions) in and out of my current lists on a weekly basis as I do my weekly review and plan for the use of discretionary time during the upcoming week based on what is on my calendar, and the priority projects that I have to move forward.
Yes, this cuts down on the number of next actions I have to choose from in a given context, but I've found that by doing this I actually complete my projects faster than if I leave all of them as current/active.
I suppose it is not strictly GTD, though in several places David does mention that the ability to say no or at last to renegotiate commitments and move things to someday maybe is a critical skill.