This is one of my current favorite sayings especially since we have just released Gist in public beta.  Because on-demand software can be accessed, tried, and consumed with relatively little front-end involvement by the company or its staff, support has become the sales function.

Knowing when, where, and how to engage is critical as is doing so without being to "salesy" or self-promotional.  Answer people's questions, address their concerns, and let the product do the marketing for you.

Doing this requires not only a singular focus on users and user experience but having the infrastructure in place to hear what is being said (reference listening framework), a system to manage incidents/tickets, and the technical expertise available to quickly address questions, fix bugs, and even take ownership of a feature request and deliver it for that specific user.

If you nail it, word will spread about your attentiveness and laser focus on the end-user.  This isn't really an earth shattering concept as world-class companies have known for many years that you can truly differentiate from your competitors with outstanding customer service.  You make it an afterthought at your own peril.