Seth Godin makes a great point in his blog yesterday, Which comes first, the product or the marketing? His point being that if you don't do your marketing homework first, then the anything you create is little more than the desires of the producer, not the wants and needs of the consumer.
And he is absolutely correct, marketing must come first! The best Entrepreneurs have a vision of what they want to create, but then they bounce that vision off the existing market place to see what the demand / need is. Too often, less successful business people think that if they produce a good product, consumers will want it. Unfortunately, that isn't always the case. The adage, "If you build it, they will come" rarely works (except in movies about Iowa corn fields and baseball). The motto in the business world, should be "if they want it, then you build it".
Demand is the engine of the marketplace. With no demand, there will be no profit potential. With no profit potential, there is not need for the product. Marketing's job is first to find out if there is a demand. And if there is a demand, then figure out what the demand is specifically for. And if there is no demand then either Marketing creates demand, or the project should be shelved.
The bottom line is, there have been lots of great products that failed because of a weak or no marketing plan, but there have been quite a few very few weak products that have been huge successes with a great marketing strategy that worked.
Do your marketing work first, within the 'scope' of what you want to create. Then build to fulfill the demand that Marketing either found or created!