TI also has an I2C-Bus and SMBus Voltage Level-Translator.
http://focus.ti.com/docs/prod/folders/p … a9306.html
we have used it as an i2c level shifter.
we have also used max3371 for rs232 compatible level translation.
both products are doing their jobs very well. but max3371 [2.2$] is expensive than pca9306 [0.90$]. if you need voltage level conversion for i2c purposes i suggest pca9306 since you need two max ic’s, if you use 
we do not try pca9306 with uart.