I am having a hard time finding any posts by someone doing it. Surely someone has. I am having trouble with it and going back and forth between a MEGA, which works, and the Fio, which doesn't, the differences are that it is on pins 20 and 21 on the MEGA versus 4 and 5 on the Fio and VCC is 5v on the MEGA and 3.3 on the Fio, but I have also tried it with an external (to the Fio) regulated 5v and still have trouble. It is a compass and I have moved the specific component back and forth. I have rewired a few times. I am pretty sure I have it right. I am also using Servo2 and NewSoftSerial on the Fio. you can't use the regular Servo library with NewSoftSerial or it glitches the PWM on every receive.
The Fio uses Analog Pins 4 and 5, not digital for I2C.
I don't know whether to laugh or cry. I don't know how many times I looked at the hardware page and didn't notice that there was some text above the place where it says that I2C is on 4 & 5 and that text says some analog pins have special functions. DOH! I wish they would use A4 and A5 to refer to them. I think all the other pins are referred to exactly as labeled.
Thank you - even unsoldered, it started working when I switched over to those pins. And it works on 3.3v or 5v. I am going to leave it on 3.3v.