In Berlin, an office building is used as an 18x18-pixel display in Blinkenlights. This is nothing new; somewhere there's a Web page showing half a dozen cases of office buildings being used as displays. What I particularly like about this one, though, is that it's interactive, and accessible to the public. You can dial the computer on your cell phone and play pong (against the computer or against another human) on the building face. You can also create your own little animated movies and send them to the sysadmins, who will put them in the computer and give you an access code; when you dial up Pong, you can then enter the access code and play your movie instead of playing Pong. The people in charge recommend using this to send love letters. Ah, geek love. There's also a Webcam you can use to see what's displayed on the building right now. (At the moment, nothing; it's morning in Berlin, and everything looks gray under a blanket of snow.)
The other recent noteworthy building hack that's been brought to my attention is this Lord of the Rings hack at MIT. Nice work. Until I saw the bend in the material, I thought for a moment that it had been painted onto the dome.