I use the same setup and I2C accessories for both mega boards, and one works, the other doesn't. I've tried it multiple times, and with different I2C devices, and its always the same, simply doesn't communicate with or detect any i2c device on the one mega.
I suppose I could try hooking up a scope to the suspect mega and see if its puts out clock or data. Maybe its dead as a doorknob.
The mega works fine for all other uses, so its not garbage, but frustrating that it was the first Arduino I ever purchased, and it caused me so much grief with i2c. I kept asking myself what I was doing wrong when it was the only Arduino I owned. Then I tried 2 other arduinos, and experienced no problems.
Just thought i'd ask in case there was some jumper or hidden config that could be off.