The Xmos chips are pretty cool. I've been using them on and off for the last couple of years.
Xmos have found their niche market in audio processing, lots of design-ins for ipod docking stations, USB audio and AVB Ethernet. So the they would make a great shield for the Arduino.
There's already been several attempts at building a Xmos shield for the Arduino:
https://www.xcore.com/projects/mosduino-xmos-based-arduino
https://www.xcore.com/projects/xarduino