Thanks all. I've been struggling with an otherwise excellent Mega 2560 R3, that for some reason doesn't communicate with any I2C device. I now know that the devices, a compass and lcd work perfectly with my Duemilanove. That also verifies their address. When I run any of 3 I2C scanners on my Mega, nothing shows up for the I2C devices. I have another Mega on order, and perhaps it will be the same, or it might work. I've tried a variety of values of pull up resistors, and also no pull ups, and still get no I2C function on the Mega. I checked the output on the Mega's I2C pins with an oscilloscope, and there is certainly a 0 to 5v signal coming out of both SCL and SDA on the Mega. It all works so slick and easy like everything else with the Duemilanove, just not with the Mega 2560 R3.
For fun, I am going to try the other two SDA and SCL pins. Can any harm arise from accidentally swapping the SDA for the SCL? I'm thinking no, as its just a zero to 5 volt square wave on either. They don't appear to be labled on the added pins next to AREF, so picking the wrong one on this attempt is 50/50.