In fact, I have a Mega2560.
I try accessing a Sonar. Power comes from USB and there's only the arduino board and the sonar connected.
I don't think it is a board problem and Wire and Serial seems to work fine, but not together.
More info about further investigations I did :
I tested this code : http://arduino.cc/en/Tutorial/SFRRangerReader
(which should work)
This part :
Wire.beginTransmission(112); // transmit to device #112 (0x70)
// the address specified in the datasheet is 224 (0xE0)
// but i2c adressing uses the high 7 bits so it's 112
Wire.write(byte(0x00)); // sets register pointer to the command register (0x00)
works fine and I still have Serial if I only execute this part.
But as soon as I execute Wire.endTransmission() after this code (end of step 1), I loose Serial communication.
I tested the return value of Wire.endTransmission() with a DEL but this function seems to return 0, so it seems it executed just fine...