Depends on your audience.
If you’re running a business with users that are technically astute, then it’s more like the 99/1 rule. Businesses like Brightkite built out their product by tapping into collaborative users and listening to everyone may prove to be more important than someone with a more traditional online business model. Older models (like a job board) are more likely to fall into the squeaky wheel syndrome. Listening to one customer will more than likely create unnecessary change for the rest of the customers; thus alienating them. Product managers get trapped in this cycle often, feeling that they have to appease every customer. (Jason Fried of 37 signals has a lot to say about this and appears to have more clients that are dissimilar to themselves, since they encourage online businesses to say ‘no’ to change until the calls for change are verified across multiple customers.) In fact this is usually the case. There is a great disparity between clients and businesses that are creating the campaign/website/application with respect to online expertise. Many clients are still trying to shoehorn things like PR into Twitter. Bad. Bad . . .Bad.
For those that have a technically challenged customer base, I would lean more toward Fried with the question of; what do 60% of your users need? The answer to this question with a usability expert (not Jakob) in tow, will get you a great foundational site to build your business on.