The tutorial pages at http://arduino.cc/en/Tutorial/MasterWriter
incorrectly say that AIN4 is SCL and AIN5 is SDA. Consulting the ATmega328 datasheet, they should be swapped. The correct pin assigments are:
PC5 (ADC5 / SCL)
PC4 (ADC4 / SDA)
The particular affected tutorials work as expected if followed, making this a sneaky error: if you use them as a reference to interface to another I2C device (not an Arduino), you will fail.
Also, the documentation for the Wire library in general is scant and frankly insufficient to use for troubleshooting. There is more thorough although not easily navigable documentation on the Wire library at http://arduino.cc/playground/Main/WireLibraryDetailedReference
that would be much more likely to be discovered and read if it were integrated into the main website documentation.
I would be happy to help with at least some of this if I have access to edit or at least suggest moderated changes to the main website. As it is when I try to do so I am prompted to log in even though I already am, then told my username does not exist.