Ethernet shield vs Wifly shield
Yes, you can use the wifly shield. However there are a couple of things to bear in mind. The most common ethernet shields are SPI based whereas the wifly is uart based. Your throughput will be reduced when using the wifly. 

Also note that the hardware UART in the arduino is used for programming sketches and communicating with the pc, so you're going to have to use software UART if you want to use both at the same time.