Apple also filed the mark with the US Patent and Trademark Office on November 12, 2008 and claimed a priority date of May 13, 2008.
The scope of the application is as broad as you might expect, but offers no real clues as to Apple's future directions:
"Computer hardware; computer software; computer programming software; computer operating system software; computer development software; computer utility software; computer software to develop other computer software; handheld and mobile digital electronic devices for the sending and receiving of telephone calls, faxes, voice mail, electronic mail, and other digital data; MP3 and other digital format audio players; handheld computers; personal digital assistants; electronic organizers; electronic notepads; magnetic data carriers; telephones; mobile phones; computer gaming machines; videophones; cameras; computer programs for personal information management, database management software, electronic mail, and voice mail; messaging software, paging software, database synchronization software, computer programs for accessing, browsing and searching online databases; computer software and firmware, namely operating system programs, data synchronization programs, and application development tool programs for personal and handheld computers and mobile phones; computer software and computer peripherals for communication between multiple computers and between computers and local and global computer networks; computer software for communication between computers and home entertainment systems; multimedia computer software for the reproduction, processing and streaming of audio, video, and other digital content; computer hardware and software for data backup; computer hardware and software for protecting, restoring and recovering data; computer memory hardware; computer disc drives; optical disc drives".
What's in there that's not clearly covered by existing products?
Handheld and mobile faxing? The mobile side is taken care of by Mac OS X's faxing capability on MacBooks. As far as I know the handhelds (iPhone and iPod touch) can't send or receive faxes unless you use one of the web-based fax services.
What's my theory about the trademark application? Please read on.
As for "personal digital assistants; electronic organizers; electronic notepads", I'd suggest that's a reference to various minor capabilities of the iPhone and iPod range.
So what's the motivation for trademarking OS X?
Commentators are coming up with all sorts of speculations, including the old chestnut of OS licensing - if the operating system is sold with a Mac, it'll be Mac OS X, but if it's on another company's hardware then it's just OS X. I don't think so.
My interpretation is simple. The reason you register a trademark is to stop other people using it, or a mark that's confusingly similar.
Apple's Mac OS trademark was registered in 1996. Once the company began using OS X to describe the variant of its operating system for the iPhone (and later the iPod touch), it made sense to seek protection for that mark as well.
Filing the OS X trademark makes sense in terms of what's already happened - there's no need to hypothesise about current or future plans to justify it. Remember Occam's Razor.