NEW YORK (CNNMoney.com) -- Better watch your back Exxon Mobil.

No, I'm not talking about tough competition for the oil giant from rivals like Chevron, Royal Dutch Shell and ConocoPhilips.

Exxon Mobil (XOM, Fortune 500), which is currently the most valuable company in the U.S. with a market capitalization of $286 billion, has to listen for the footsteps of Apple.

Of course, Apple (AAPL, Fortune 500) doesn't compete with Exxon Mobil -- although given time I'm sure that someone will find a way to create an app that turns your iPhone into an oil rig.

But Apple now is the second most valuable company in the land. It has passed Microsoft (MSFT, Fortune 500) and Wal-Mart (WMT, Fortune 500) this year and now trails Exxon Mobil's market value by only about $50...