Hi, just received my Mega and it is running everything OK except the 20x4 i2c serial LCD.
I did some searching and tried everything I found, but nothing is working.
I used the i2c Scanner sketch and it returns "No I2C devices found."
Any suggestions? The Scanner works fine on a Uno and detects the LCD at 0x27.
On the Mega, SDA and SCL are on pins 20 and 21. These are different pins than on the Uno.
Is your connection to the LCD on the correct pins?
Ahhh, great, thanks. It is working fine. I didn't think to look for those on the Mega. I just assumed it would be 4 & 5 as I remember reading something about the chip being hard coded for those pins. Did not occur that the different chip might have different pins.
Looking at the Mega now, I can see they have dedicated pins for SDA/SLC. DUH! 
... as I remember reading something about the chip being hard coded for those pins.
The Mega uses a different chip from the UNO and the I2C (actually TWI) is hard coded to different pins.
Don
Assumption is the mutha of all f$@kups!