If 37 signals had not written SVN blog and released rails, do you think as many people would have come? I would put that also into the "marketing" category
37s was a very well known usability consulting group for years before hiring DHH and building Basecamp. Not only is SvN good marketing, but 37s had the kind of powerhouse targeted niche brand that is nearly impossible to obtain.
If anyone cares for ancient history, one of the biggest attention getters for 37signals was their manifesto which made up the entirety of 37signals.com before they revealed what they actually did. I still remember seeing it for the first time and it went huge (yet I can't remember HTF things went viral online back then... :)).
I disagree with the "nearly impossible to obtain" concept. That's suggesting they're something special or inhuman that your average person can't obtain, while it's quite the contrary. 37S is a great example f what busting your ass can accomplish; which I've found is the key to most cases of success. A good idea helps, but until you really take it to heart and live with the devotion it takes to succeed, you won't get very far with any product, no matter how revolutionary.
Statistically speaking, hiring a good marketing person is going to be much more effective than a long term strategy of becoming a thought leader in your target market (how many target markets have thought leaders anyway?) and then exploiting your massive popularity for an instant word-of-mouth network effect when you launch your first product. Even 37s themselves did not engineer this success. Instead they focused on doing what they do well. They did not get where they are by picking some outlier to emulate.