Good evening,
I am building a "weather station", well... kind of, with my Arduino Duemilanove, and as barometer I am going to use a BMP085 sensor (on the Sparkfun breakout board). This sensor is I2C device, and only works at 3.3V. I read some topics about this sensor here on the forums, but if y problem is nearly the same, my question is different 
My question:
If a connect the 3V3 output of my board to the Vcc pin of the BMP, GND to GND and the 2 I2C pins to A4 and A5, after Sparkfun it will do the job. After me, it will not work, or if it works, maybe it's going to damage my sensor, because the BMP chip will not like the 5V on the I2C bus !
I think there is a solution to my problem, but I ask you because I am not sure this is a good solution (and I'm a noob ^^) :
I would connect the 3V3 output pin of my arduino on the AREF pin, so the analog PINs, and thus A4 and A5, would have max 3.3 (well... I'm not sure about it!)).
In my sketch, I would write : analogReference(EXTERNAL) before charging wire library. And may be I would need to add a filter between the 3V3 output and the AREF to cut noise.