BlackBerry has been able to carve out a healthy subscriber base by positioning itself as an email-and-texting corporate gadget. Apple's offering is fun, cool, and resourceful, but it's a pain to peck out communiques on the iPhone.

Anyone who thinks that the iPhone can grow exponentially -- just as BlackBerry, Google, and hungry handset vets all push for growth -- is missing the ceiling. We've seen the mighty MacBook and Mac desktops peak; both had massive year-over-year sales dropoffs last quarter. Don't assume that iPhone growth will last forever.

This isn't a $13-a-month Sirius XM Radio (Nasdaq: SIRI) subscription, where it's the only game in town when it comes to satellite radio. We're talking about an aspirational device that is certainly not a cheap indulgence.    

The $28 billion question
Finally, let's not assume that $28 billion makes Apple's think tank too big to fail. As rich as the Cupertino cradler is, it still birthed Apple TV, and missed last year's consumer shift to netbooks.

Companies like Microsoft and Cisco (Nasdaq: CSCO) also have tens of billions in the bank, but that hasn't been enough to bury the blunders that found both stocks closing in the high teens yesterday.

Apple's cash would be a valuation factor if the stock were trading close to the value of its liquid investments and cash, as it nearly has done several times in the last dozen years, but it's clearly trading at a market premium these days. Tim is right that Apple's history is one of hits, but it's also one of misses.

Coming off a quarter in which the only thing going in the right direction at Apple was the iPhone, I would hate to see investors overbidding for a company that seems to have become a one-trick pony when it comes to growth.