I2c and it's pull-up resistors

I'm working on interfacing a number of different devices to my Arduino over I2c. The datasheet for the Atmel168 says that, "The only external hardware needed to implement the bus is a single pull-up resistor for each of the TWI bus lines."

So I tied both of my bus lines to the 5v supply of the Arduino with 1.8k resistors and was ultimately able to communicate with a motor controller over my TWI bus.

The part I'm wondering about is I now have a smaller sensor that runs off of 3.3v and is also capable of communicating over I2c. But I can't just hook this device up to the 5v TWI that I have working now, right?
Is it true that I can just tie both of my TWI bus lines to the 3.3v supply instead of connecting them to the 5v? Will this give me a 3.3v TWI bus that I can use with my new sensors?

Some 3.3V devices are what is called "5V tolerant", that is, they a designed for 3.3 volt but won't crash and burn if supplied with 5v.

Unless you sensor is such a devive i don't think it's a good idea to hook it up to a 5v I2C bus.

Is it true that I can just tie both of my TWI bus lines to the 3.3v supply instead of connecting them to the 5v?

Yes

Will this give me a 3.3v TWI bus that I can use with my new sensors?

Yes.

As the I2C bus drivers basically pull down then using 3V3 will allow you to communicate with both your 5V and 3V3 devices.
How did you arrive at 1K8? That's fine for a 3V3 device but a bit low for a 5V device, I would use 4K7 for a 5V device.

Thanks for the info, the 3.3v bus worked just how I expected. Success!