These tiles will sense and ID tiles that are adjacent to each other.
On the Sifteo site it mentions proprietary near field sensing technology. How does it work? Does each side of the tile have some sort of reader and communication device?
UPDATE I think that there's a proximity switch on each side and when another tile is brought close to it; the switch activates a directional radio for each tile so that tiles can transmit information. This is all just a guess.
How can you design a radio that transmits only at short distances? You don't want the signal to go further because it might interfere with other tiles communicating.