I just started with Arduino and all is going well so far. But next up is working with some parts that have an i2c interface, which I understand should work fine using the TWI. It seems to me that would be fine if the Arduino was stand alone, but during development the Arduino is connected to my computer via USB. since the tx and rx bits are shared with the USB, won't any USB activity mess up i2c comms, and won't any i2c activity get sent out the USB? Or does the USB chip somehow know that it isn't valid data to send back to the host?
I2C uses different pins than the USART. Using serial and I2C at the same time is no problem - I've got a circuit and sketch that use the USART, SPI, and I2C all at the same time with no problems.
-j
Ahhh. Thanks. I found it now. i2c is using the analog 4 & 5 pins. That will work spiffy! 
The i2c examples I found before seem to predate official Arduino support and, for whatever reason, people were using the tx/rx lines and cobbled it together themselves. I assumed :-[ that the official library did the same.