Apple has told MacCentral that CEO Steve Jobs' promise that the company will offer 3GHz G5 Power Macs this summer won't be met.

Jobs made the promise at last year's Worldwide Developers Conference, and the company's newly unveiled Power Mac G5 line up all fall short of the 3GHz goal.

"The technology challenge is bigger than we expected," admitted Apple's director of Power Mac product marketing Tom Boger.

He added: "It's actually quite simple. When we made the prediction that we would reach 3GHz within a year, we just didn't realize the challenges moving to 90 nanometer would present. It turned out to be a much bigger challenge than anyone expected."

Boger also told MacCentral that Mac users shouldn't expect to see PowerBook G5s before the end of the year because "the technical challenges of putting that G5 in a small enclosure are not trivial".

It is a similar story with the iMac G5. "The challenges are obvious when you look at the G5 and the size of the heatsinks and the enclosure; that the iMac would be a heck of a challenge as well," Boger said.