MacRumors.com

Apple previously trademarked the term "Mac Pro" in New Zealand in late 2005. On May 17th 2006, the same name was trademarked through the U.S. Patent and Trademark Office under the following classifications:

computers; computer hardware; computer software; computer peripherals; notebook computers; laptop computers; tablet computers; computer servers; handheld computers; mobile computers; hard drives; audio speakers; speakers for computers; personal digital assistants; electronic organizers; electronic notepads; magnetic data carriers; computer gaming machines; microprocessors; memories boards; monitors; displays; keyboards; computer input devices; computer cables; modems; printers; parts and accessories for all the aforesaid goods

This appears to add further evidence that Apple may be considering changing the name of the PowerMac to "Mac Pro" in the near future. Apple rebranded the PowerBook in January to the "MacBook Pro" and subsequently dropped the iBook name in favor of "MacBook".

Steve Jobs had indicated that they were moving away from the "Power" name when the MacBook Pro was first announced:

It's a new name [MacBook Pro] because we're kinda done with 'Power' and because we want 'Mac' in the name of our products."