Sometime this quarter, Netflix added its 9 millionth subscriber. Sometime in the future, it hopes to make money on its movie-streaming service.

The gap between the current success of the Netflix DVD-by-mail business and the uncertain but promising posture of the streaming initiative was evident in comments by CFO Barry McCarthy, who appeared today (12/9) at the UBS Global Media and Communications conference.

As has been customary in recent Netflix investor presentations, McCarthy portrayed the Netflix “watch instantly” streaming service as an investment in the future that, for the moment, works as a sort of “tax on the P&L.” In other words, it’s a drain on profitability for a company that reported earnings of $20.4 million for the third quarter, up 30% from a year earlier.

Still, the service is said to be attracting a growing number of users, although Netflix hasn’t specified the exact numbers. By making a number of deals this year with technology companies (LG Electronics, Roku), content partners (Starz Entertainment) and distribution providers (Microsoft’s Xbox and TiVo), Netflix has been trying to “jump-start an ecosystem,” McCarthy said.

He said Netflix isn’t sure exactly how to accurately divine the economic model that will determine when, or if, the streaming service one day might overtake Netflix’s burgeoning DVD-rental business. But he said if Netflix were to ignore the rising marketing for streaming movies to PCs and TV-connected devices, it would be at its own peril. “What we do know is that if we don’t launch it, somebody else is going to,” he said.

The instant-watch service offers roughly 12,000 titles now, an amount that has led some critics to complain it is too thinly populated to offer mainstream appeal. McCarthy said Netflix could readily add to the content reservoir tomorrow – but at a cost. “More content is about more money, mostly,” he said.

In closing comments, McCarthy said investors tend to overlook one of the key contributors to Netflix’s growth – a well-crafted user interface that lets customers browse through movies and videos. He said the cable industry’s inability to come up with a similarly graceful interface has hurt the industry’s video-on-demand business. “I would cite cable as an example of a failed interface and Apple…as a pure form example of why a great user interface drives success,” McCarthy said.