This tutorial (http://www.ladyada.net/learn/sensors/bmp085.html) recommends against keeping 3.3V and 5V devices on the same bus.
You may be wondering, how is it OK to connect a 3.3V chip like the BMP085 to 5.0V data pins like the Arduino? Isn't that bad? Well, in this specific case its OK. I2c uses pullup lines to the 3.3V power pin, so the data is actually being sent at 3.3V. As long as all the sensors/device on the i2c bus are running on 3.3V power, we're fine.
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