...our minds think differently when on software.
Runway for example. Outside of the PC, this means jotting down check lists.
On it this means constantly dancing around e-mails, rss readers, social networks like facebook and twitter and manipulating software.
The design of a to-do list of paper means you can spot entries and want to check them off or cross them out like a grocery list.
On the opposite spectrum, you check software to-do lists when you want to do something or immediately after you have done something.
Because of the difference in scale - an average barebones software to-do list is actually 40,000 feet in GTD Airspace lingo (3-5 year vision).
The Runway instead of a software productivity system is when you are not "seeing" the software as it's alt-tabbed elsewhere and you are doing something you know is your next action.
But treated this way, the Runway becomes the GTD equivalent of 20,000 feet (areas of responsibility) so it's not a runway.
That is to say, your current actions are not your current actions.
Therefore it's safe to say that in order to have a Runway in software form, Runways must be treated like programs where you are alt-tabbing around a system. The conclusion being that it's not possible to have an efficient GTD system unless you are using multiple to-do lists system and as such the more a productivity system tries to be the "only one" the more it's not staying true to the concepts of GTD. This is speaking less of how a program adopts to GTD terms but more of how GTD should be able to make a computer user productive if they are using a software GTD system rather than hardware.
Note that this applies to productive software GTD users in that they often claim GTD works for them but they often delegate half of the airspace underneath missions and visions as opposed to truly trying to understand why GTD emphasizes that airspace so much. Coincidentally, no software whether they be GTD or not have tried using the terms as a default structure often because such a simple concept is treated as "complicated" even though tags, stars, colored priorities, do it tomorrow systems are more complicated. (Note that I do not consider brainstorming sessions as counting as airspace and thus I include mindmaps as similarly being incomplete even though David Allen uses mindmaps to get his software airspace concept in place.)