What processor will the iPhone use? That's one of the great unanswered questions left in the wake of Apple Inc's launch of the iPhone this week, but a job ad on Apple's website may hint at the answer.

The open position is a networking engineer to work on the iPhone and as the ad makes clear, Apple is looking for someone who knows the Mac OS and embedded platforms, and Arm processors in particular.

An Arm chip would be the logical component for a device like the iPhone. Arm is already the dominant provider of smartphone application processors, because its chips are powerful enough to meet the computing needs of these products, while at the same time requiring little power — a critical component to keeping the phones running between charges.

If the iPhone's brain is Arm-based it will represent a new direction for Mac OS X operating system, which will power the device.

To date Mac OS has been ported to Intel's x86 processors and IBM's PowerPC, but Arm would represent a third platform for Apple's operating system.

On the other hand, it is possible that Intel or IBM could develop new processors that would meet the iPhone's requirements, saving Apple from doing the porting work to make the OS run on the new platform, said Peter Glaskowsky, technical analyst with the Envisioneering Group.

"To me the number-one unresolved question about the iPhone is: does it use an Arm with a port, or does it use some new PowerPC or x86 chip?" he said.

Glaskowsky said that even if Apple is looking for Arm skills, that doesn't necessarily mean that Arm will be powering the iPhone's operating system. "I wouldn't read too much into that," he said of the ad. "If you think about most modern smart phones, it's very common for them to have four processors in the device.

Apple could be using an Arm chip to do something like voice compression or to simply process wireless networking signals (Apple's ad is for a "Bluetooth/WiFi" software engineer).

"It may have nothing to do with what's running Mac OS on there," Glaskowsky said.