This thread is surprisingly active for one started in 2003...
@Jan Ernest, I understand the desire for clear-cut answers to GTD questions but I've learned over time that GTD isn't like an equation where you can "solve for X". There usually isn't one right answer to a GTD question, even though we in these forums sometimes succumb to that belief (and I'm no exception).
I can tell you with certainty that David Allen defines a "next action" as a physical, visible activity that you can do in one sitting. It's in the book. But what that means is open to interpretation.
Moreover, I'm not sure anyone can accurately say something "cannot" be a next action. My guess is that even David Allen himself doesn't do GTD "by the book" 100% of the time.
I've found that experience is a better guide than anything else when it comes to GTD. If "make a decision" as a next action works well for you, then there's nothing to change. If you're more like me and find yourself avoiding such a next action because it's too vague for you, ask yourself "how" you will make that decision and what person/place/tool you'll need to accomplish it. Then capture it in the appropriate list and do it when it's appropriate. If my advice doesn't work either, keep experimenting until you find something that does.
I think the question of what are the most appropriate actions for making a decision is beyond the scope of this forum, because the answer depends on your thinking style and the type of decision you're trying to make. Making a large purchasing decision in a corporate enviornment, for example, will be a lot different from buying a new shirt.
Ironically, after I got done blowing my stack a bit with
@mcogilvie (for which I apologize, mcogilvie) I realized that a sometimes when I elaborately plan how to make a decision, I end up ignoring the plan making a snap decision based on my intuition in the moment. That's OK too. GTD is meant to be an enabler, not a straitjacket.