Recently I have been experimenting a lot with I2C. My A.I. project uses the DS1307 RTC and a work project uses several I2C ADC's.
Normally the Arduino Wire library works fine with the DS1307 RTC but it does not play well with 3.3V @ 8MHz devices such as my Micro Magician and does not support fast mode (400KHz). It also has some compatibility issues with some devices such as my ADC's because it does not support a repeated start condition.
This new library is easy to use, has some cool new features such as the ability to scan the I2C bus for addressable devices and solves all my problems. I suspect it will be incorporated into the Arduino IDE at a later stage but for now you can read about it and download it from here: http://dsscircuits.com/articles/arduino-i2c-master-library.html
The library also claims to be smaller and faster than the original Wire library.