As reported yesterday, the NPD Group estimates that up to eight million people are willing to pay $10 or more per month for an Apple cloud-based music service -- if one actually launches at some point. Here's what makes that number so staggering:

-- Apple's sign-up rate could be over 10 times better than the nearest competitor. To put it in perspective, the eight million subscribers forecast by NPD would represent 2.58% of the U.S. population. At 675,000 subscribers, Rhapsody currently accounts for about 0.22% of the U.S. population. Spotify has about 500,000 paying subscribers (prices vary by markets but are comparable to those of Rhapsody). The service is available in seven countries with a combined population of about 210 million people, giving Spotify a 0.24% sign-up rate. (Spotify has about 6.5 million users of its free, ad-supported version. Those users do generate revenue through advertising deals. For comparative purposes, however, Spotify's non-paying users are excluded here.)

-- Apple has a large numbers of potential captive customers. NPD pegs the number of U.S. iTunes users at 50 million. A portion of this group has already given Apple their credit card information to purchase music, video and apps at the iTunes store. Thus, Apple has a big advantage over its competitors in signing up new users.

-- More than 19 million people in the United States are currently potential mobile users of an Apple service. According to an estimate by AdMob, in May there were about 19 million devices in the United States powered by iOS, Apple's operating system for the iPhone, iPod Touch and iPad. By the debut of a service, which would likely attract mobile users, millions more cloud-enabled mobile devices will be on the market.

-- The company has a proven ability to offer products and services that resonate with consumers. Apple was wise to stay out of music subscriptions thus far. As it has existed for the better part of a decade, the subscription service has not appealed a wide audience. But with changes in services and technologies, the old limitations of DRM downloads have given way to the freedom of streaming to a multitude of popular devices. Simply put, the subscription service hasn't yet taken off because Apple hasn't yet got into the game.