Hi, I've got an Arduino Mega, and I'm trying to use the http://arduino.cc/playground/Main/I2cScanner from the playground. When scanning, it prints "I2C Scanner" and "Scanning..." over serial, never does anything after that. Through a bit of debugging, it hangs up on Wire.endTransmission(). The Mega has nothing attached to it at all apart from USB. I've run the same code on a Duemilanove with nothing attached and it works fine, scanning through all the addresses.
I've tested the SDA (20) and SCL(21) pins with a scope, and if I set them as outputs and toggle them on and off, they both work fine. When setting pin 20 to input and writing high to it however, it stays low. Any ideas?