A good, cheap and safe way of level-shifting I2C signals is explained in 'Phillips AN97055', available at many sites. (EG www.adafruit.com/datasheets/an7055.pdf)
All you need is the 5v and 3.3v pullups and 2 n-channel enhancement mode FETs. I use 2n7000, 50 for £2.00 on eBay.
SparkFun and others have a small product that I believe would help handle the I2C logic level converting nicely and then would also provide the improvement to be able to safely using I2C on any number of 3.3V and 5V devices. This could be helpful to use a "backpack" to run the LCD display by I2C and use the pins saved in moving that over for other purposes. I believe this do basically what PeteS is talking about above.
- SparkFun: SparkFun Logic Level Converter - Bi-Directional
- AdaFruit: [www.adafruit.com/products/757 4-channel I2C-safe Bi-directional Logic Level Converter]