Underestimating Apple (AAPL) is dangerous. To illustrate why, I dug into a report over the weekend by one of the best research analysts on Apple, Toni Sacconaghi of Bernstein Research. The thoughtful Wall Streeter noted Apple’s intentions to double services revenues in four years and asked how it could accomplish this feat. After accounting for expected growth, Apple would need an additional $13 billion in annual revenue from services, Sacconaghi writes. He concludes that only acquisitions will get it there, just as Aaron Pressman of Fortune astutely surmised last week.

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First, Sacconaghi notes how relatively good Apple’s services business is. It counts only its take of App Store and related revenues, a high-margin 30% commission. Similar companies account for the whole purchase price customers pay. This means that buying even as big a content player as Netflix ($9 billion in revenue, accounted for the non-Apple way) won’t cut it.

The analyst thinks Apple has ample untapped opportunity if only it would market its services more aggressively. “What is striking to us is that perhaps in the spirit of upholding its privacy tenets, the App Store is much more ‘pull’ than ‘push,’ and it is a transactional marketplace, not a real ‘community.’” He cites Apple’s unwillingness, unlike Facebook and Amazon, to implore users by email to come back, to offer tools to create interest lists, to provide user-friendly recommendation engines, and so on.

Sacconaghi also sees opportunity for Apple to snap up small game developers because gaming is so popular in the App Store. In terms of traditional content development, he thinks it is “increasingly unlikely” that Apple could set up its own cable-like service and instead will be forced to “aggressively acquire (or more likely) license content.”

Watching Apple maneuver will be great fun. Just don’t underestimate it.