> Research first, and make damn sure there's a viable market there before you sink time into the idea. This is essentially what a business plan is, a documented view of the market and how you're going to make money from it.
I'd disagree on a number of points. A business plan is something that gives you an idea of where you've been, where you're headed and what the strategic objectives over the next period (normally a year) are going to be, who's responsible and how that will be executed. For a first year, it's largely a case of documenting what you're going to do and how you're going to do it, feeding in from market analysis and all the other researchy stuff before you start.
The research is a fundamental part of shipping - you can split it out if you want, I don't, mainly for two reasons:
1. You don't actually have a startup until you ship.
2. See 1.
Likewise, writing a business plan without a business isn't quite a fools errand, but isn't the best use of your time because at that point in time you do not have a business, and the minute you do everything will change.
It's great to do the research, in most cases it's essential, the things you're describing (view of the market and how you'll make money out of it) are things that feed into the plan and are actually more important than the plan, not the plan itself.