I noticed an error in this page: http://arduino.cc/en/Main/ArduinoBoardDuemilanove.
At the end of the Input and Output section, it's written that the I2C pins are 4 (SDA) and 5 (SCL) but in reality they are A4 and A5, like what stated Arduino Uno datasheet.
Actually there is a little paragraph above that line saying the that the analog pins have some specialized functionality such as I2C, AREF, etc. but it's somewhat misleading considering that the page is supposed to be a quick reference.
I suggest to change the line
I2C: 4 (SDA) and 5 (SCL). Support I2C (TWI) communication using the Wire library.
I2C: A4 (SDA) and A5 (SCL). Support I2C (TWI) communication using the Wire library.
just like the Arduino Uno page. ;)
Fixed. The old language was from before we referred to the analog pins as A4 and A5, it should be clearer now.