Sorry to spam the thread, but I just came across the Rugged Circuits Audio Sheild while reading up on another of their products.
It has 12/44.1 input and output; unbalanced mic in, line in and headphone out; microSD storage, and an on shield ATxmega32A4 microcontroller to control it all, offloading that from the main Arduino procesor. Which seems like a pretty good deal for USD 34.95; I might get one myself.
I would suggest ensuring your proposed shield is either much cheaper, or much better.