The Whereabouts Clock -- Background

I've been a fan of the Harry Potter series since the movies first hit the big screen in 2001. Since then I've read all the books, and I can't wait until the final movie is out. In a couple of the books and in the movies there are a few references to a magical clock at the Weasley's house:

The first specific description of the family clock is from Goblet of Fire (pages. 151-153):

Mrs. Weasley glanced at the grandfather clock in the corner. Harry liked this clock. It was completely useless if you wanted to know the time, but otherwise very informative. It had nine golden hands, and each of them was engraved with one of the Weasley family’s names. There were no numerals around the face, but descriptions of where each family member might be. “Home,” “school,” and “work” were there, but there was also “traveling,” “lost,” “hospital,” “prison,” and, in the position where the number twelve would be on a normal clock, “mortal peril.” Eight of the hands were currently pointing to the “home” position, but Mr. Weasley’s, which was the longest, was still pointing to “work.” . . . “Oh your father’s coming!” she said suddenly, looking up at the clock again. Mr. Weasley’s hand had suddenly spun from “work” to “traveling”; a second later it had shuddered to a halt on “home” with the others, and they heard him calling from the kitchen.

It appears again in Order of the Phoenix (pages 471-472):

”And Dumbledore – what about Molly?” said Professor McGonagall, pausing at the door. “That will be a job for Fawkes when he has finished keeping a lookout for anybody approaching,” said Dumbledore. “But she may already know . . . that excellent clock of hers . . .” Harry knew Dumbledore was referring to the clock that told, not the time, but the whereabouts and conditions of the various Weasley family members, and with a pang he thought that Mr. Weasley’s hand must, even now, be pointing at “mortal peril.”

And in the Half-Blood Prince (pages. 85-88):

She turned to look at a large clock that was perched awkwardly on top of a pile of sheets in the washing basket at the end of the table. Harry recognized it at once: It had nine hands, each inscribed with the name of a family member, and usually hung on the Weasley’s sitting room wall, though its current position suggested that Mrs. Weasley had taken to carrying it around the house with her. Every single one of the nine hands was now pointing at “mortal peril.” “It’s been like that for a while now,” said Mrs. Weasley, in an unconvincingly casual voice, “ever since You-Know-Who came back into the open. I suppose everybody’s in mortal danger now . . . . I don’t think it can be just our family . . . but I don’t know anyone else who’s got a clock like this, so I can’t check. Oh!” With a sudden exclamation she pointed at the clock’s face. Mr. Weasley’s hand had switched to “traveling.” . . . Harry saw Mrs. Weasley glance at the clock in the washing basket as they left the kitchen. All the hands were once again at “mortal peril.”

I've seen a few attempts to recreate this clock in a real and physical sense. This person created a very good looking clock that updates the locations via keywords in Twitter feeds. Microsoft apparently is making an attempt as well, but theirs doesn't look much like a clock.

I wanted a nice looking clock that would automatically track my family without having to update locations manually. My wife and I both have iPhones, so this should be easy, right? It turns out that what I thought was hard (the location data) is actually pretty easy.