Building a 3-axis acceleration sensor is just a bit harder than building other i2c sensors (because accelerometers chips have smaller pins), but it is definitely possible. If you want to learn how to build one, or even if you are just curious about how the commercial acceleration sensors work, you will find the web page that describes a home-brewed acceleration sensor interesting.

The web page also shows how to connect I²C chips that run on 3.3 Volts to the NXT. This technique may be useful for a variety of I²C projects.