I agree, for most people paper is a good starting point. However, if any of the following apply to you, then you may eventually be better off going electronic:

1. Most of your NAs require an electronic means to close them (i.e. are your reference material and tools electronic?)

2. Most of your NAs "arrive" via electronic means (e.g. electronic calendar, e-mail and/or project management app)?

3. You already manage things pre-GTD electronically, and are you reasonably happy doing things electronically

That said, even if going electronic is called for, do not fall into the trap of trying to make your entire system electronic. For most people, it will be impossible to go 100% electronic (think of reference materials here . . . are you really commited to scanning everything?). Since we are talking about starting out with GTD, I will offer my heretical approach to implementing GTD.

I suggest taking the time to research GTD before committing to it. Read the book, read GTD related stuff online, and then experiment with the concepts. Try applying GTD to one small facet of your life, and do it with paper only (i.e. literally by the book).

I suggest starting with your snail mail, both at home and at work. Set a time limit on this, but do give yourself the time to work through at least one billing cycle (so probably a month). Keep notes on what worked, what gave you problems, and what was frustrating. Now you are ready to plan your GTD implementation.

Review your notes, and try to identify what you think you will need to take GTD to the next level. If it is too overwhelming, take a smaller step, and add e-mail to the GTD mix. Wash, rinse, repeat until you are completely "in" GTD-wise. However, this incremental approach will take much longer than the approach advocated by the book. In any event, take your lessons learned from GTDing snail mail, and completely re-evaluate how you setup your contexts.

Think about your GTD experience as comprehensively as possible, especially about the "super" contexts of Agenda, Errands, Home and Work. You may be better off matching a particular GTD mode (paper vs. PDA vs. desktop application vs web application) to each of these super contexts. Then consider what (if any) interfaces there may need to be between them. Weigh the effort it will take to deal with each interface; and, if the effort is too great, pick the better mode of the two connected by the interface, drop the losing mode, and elimate the offending interface.

I would up with paper for Agenda items, PDA for Errands, and a set of desktop applicaitons for Home and Work.