I would say you don't need brilliant marketing before you can make a product. Brilliant marketing can maximize the money you make off an idea, but you can make a great product with a small market. 37signals is a great example. They build products for them, since they know what they want.
Of course, this doesn't work in a lot of situations. People who don't know how to write software need software too. To start a startup though, all you need is an idea that some people might think is ok. If it really is good, marketing and business will come later, after you've developed the product.
I think that 37signals is the wrong example to pick here, they are where they are because of great marketing. There are loads of sites that do ajaxified to-do lists, a good hacker could probably code up their ta-da lists in a weekend. The reason they are popular is because they have attitide and great marketing skill. Just look at the number of submissions to YC news from their blog.
And no ROR has nothing to do with this - you could implement their site in Cobol if you wanted to and the average customer wouldn't know the difference.
I agree that a lot of their success has to do with great marketing, and I've never said that marketing isn't important. I believe, however, that 37Signals did very little market research when they started. They made products and then figured out how to get users, not the other way around. Now they're successful and can use that success to grow even faster.
You bootstrap with great products, not great marketing.
Of course, this doesn't work in a lot of situations. People who don't know how to write software need software too. To start a startup though, all you need is an idea that some people might think is ok. If it really is good, marketing and business will come later, after you've developed the product.