Not being a Harry Potter fan the "wearabouts clock" has passed me by, but this DIY location-indicating replica is impressive enough even without the back story. The concept is that rather than display the time, the clock's numerous hands show whereabouts each family member is, according to various locations (or situations, such as "mortal peril") around the clock face. As for the magic inside, our old friend the Arduino and a hacked Linksys router monitor Twitter for location updates.
The first generation of the system attempts to recognize the first few words in the tweet - it monitors a different Twitter stream for each hand - and then shuffles the indicators around to the appropriate point. If the message can't be interpreted that way, it points to "read me".
Video demo after the cut
Future versions, however, may be tied into Latitude so that location can be automatically figured out, and the somewhat jerky servos may be replaced by stepper motors for a more clock-like movement. I might be tempted to throw in a small display that would actually show the tweet itself, but maybe they don't have LCDs in the Harry Potter universe; having never read the books I wouldn't know.