I2Cdev #include weirdness
The Arduino IDE means well, really it does. It copies your source, the device core, and any libraries used by your source all to a separate directory so that it doesn't poison any of their original locations unintentionally.

But of course, there's something missing from that list. The IDE doesn't copy libraries used by other libraries. Therefore if you don't include Wire.h in your .ino file then Wire will be missing from the build and you get the error observed. Cracktastic.