Apple (AAPL) is moving into big business with tremendous speed. Most of the Fortune 500 companies are either testing iPads and iPhones or allow them for use by employees.

One of the biggest companies to embrace Apple may surprise you. ZDNet reports that IBM (IBM) has 30,000 employees with iPhones, 10,000 using iPads and 10,000 with MacBooks. That's according to a presentation at a recent Macworld conference.

IBM now allows 50,000 Apple products into its workforce. Unimaginable. This may be the largest Apple deployment out there, writes Eric Lai at ZDNet. There are still 30,000 IBM employees using the BlackBerry line of phones from Research In Motion (RIMM), but as you can imagine, that group is shrinking.

It wasn't long ago when using a Mac or an iPhone for business was practically unheard of. Information-technolo​gy departments were loathe to begin working with an additional operating system and hardware platform. Windows was enough work as it was.

But employees began asking for it. And businesses found that their customers were using Apple tools. Finally, Apple began making its products more enterprise-friendly.​ Soon enough, it wasn't all that difficult to bring on an iPad or an iPhone. The ability to develop customized business apps added to the appeal.

But there's one caveat here. Most of the Apple products at IBM were brought in by the employees -- not purchased by the company, Lai reports. That's an important distinction. Still, the high employee interest will be a factor the next time IBM makes a big hardware decision.

Forrester thinks Apple will grow its sales to businesses by 58% this year to $19 billion. Next year, enterprise sales could hit $28 billion. Forrester expects sales of Microsoft (MSFT) Windows to fall by 3% this year. (Microsoft owns and publishes Top Stocks, an MSN Money site.)

Many investors still dismiss Apple as a presence in the workplace. But this is an area with nearly as much potential for the company as expansion in China -- and it should be taken seriously.