I did a bit more work on this and it appears the only address that won't work with my sketch is 0x20. 0x21 to 0x27 do work, I tested each one by making both h/w and corresponding s/w changes. The pullup resistors aren't the problem as I thought. I shorted them out to make address 0x20 but same problem. I'm thinking maybe an address clash somewhere but it's not obvious. No other i2c devices on the bus. Software is my own arduino sketch running on an Etherten (atmega328) and compiled with latest arduino 1.0.1. I tried compliling with ver 1.0 but same problem. LCD i2c Library is Francisco Malpartida LiquidCrystal_V1.2.1