One of the big shifts in the marketing paradigm today is in the relationship of customers to the sales process. The broadcast marketing model was all about persuading customers to buy (by interrupting with effective, outbound messages). The P2P marketing model is based on inspiring customers to both buy and advocate your brand to others (by providing relevant products, service, content, and dialogue).

Last week’s BusinessWeek cover story showed the new model at work – in the nascent growth of Mac computers in corporate environments. The Mac may finally be getting some traction in companies outside of the traditional niche of design and communications firms.

The intriguing part of the story is that this growth is happening despite the fact that Apple has no corporate sales force. This is intentional. Steve Jobs has argued that companies can succeed by focusing on corporate or consumer buyers, but not both. (Agree? Comments? Counter examples?)

What is driving the Mac’s entry into the corporate environment is that managers are finally giving in to growing requests from employees to bring a Mac into the office place. Companies like IBM and Cisco are allowing pilot programs where a few dozen employees are allowed to switch from Windows to Mac OS, in order to gauge the impact on the organization. (Has any manager ever been faced with employees clamoring to allow Windows into the workplace?)

Part of this shift may be driven by the catastrophic roll-out of Microsoft Vista last year (like many, I’m sticking to my XP guns). But the Mac “pilot” programs are also testament to the power of inspiring a community of customers to support your brand, rather than persuading them to buy from you because they have no other viable choice.

Not long ago, market competition was seen in terms of functional product categories. Marketing textbooks told you that Barnes & Nobles competed with Borders (bookstores), Delta competed with United (airlines), and McDonald’s competed with Burger King (burger & fries). Many marketers persist in sprinkling these competitive sets onto their 1960s-era positioning maps.

But a huge shift has taken place in consumers’ choices and the fluidity with which they move between them. Nowadays, McDonald’s is as likely to compete with Starbucks for a diner’s time and dollar as with another burger joint (“Do I want a quick bite with fries? Or maybe a sandwich and latte?”). New York–to–DC commuters are not just picking between Delta and United, but considering the Acela train as well. And book lovers can skip the bookstore altogether for Amazon.com (or skip the book itself, opting for RSS, podcast, or a Kindle). Companies compete not in functional product categories, but within the contexts of consumer needs and experiences: a casual meal, a productive commute, a thoughtful read.

This was the argument put forward by Sirius satellite radio to the Dept. of Justice in its current bid to acquire XM, its sole competitor in the field of satellite radio.

From a traditional view of competition, this would be an open-and-closed case of free trade infringement. If Sirius buys XM, it will be the only satellite radio game in town. Zero competitors equals zero choice for consumers, and an anti-competitive monopoly, right?

But Sirius argued that its real competition is not XM, but all the other options consumers have for incorporating audio and media into their drive time: from hi-def radio, to mP3 players (my 2007 car came with a plug where my iPod fits right in), to cellphones and backseat DVDs.

Today, the DOJ announced that it had accepted Sirius’s argument and is greenlighting the acquisition (still to weigh in: the FCC). Anti-trust regulation is still vital to preserving competition, but for many areas of the market, the scope and dynamics of competition have expanded greatly. Watch out Stabucks: even McDonald’s is starting to offer lattes of its own.

-David Rogers