I have begun to experiment with my new Arduino Ethernet Shield. The shield itself had a built in miniSD card slot so today I went out and got a
4GB Sandisk microSDHC Card from Staples so that I could see how this works. I haven't gotten anywhere yet with the SD card but I guess it could be extremely useful for holding a database of sensor data or something like that. I have done some research so far and I have found that the Arduino uses digital pins 10, 11, 12, and 13 (SPI) to communicate with the W5100 on the ethernet shield and so these pins cannot be used for general i/o.
I also just want to add that I was very impressed with the SD card because it boasted the following on the front of the package: