This is the interesting question asked by Steve Huff on his blog. As a matter of fact, he took a Leica Noctilux ASPH f/0.95 lens on both the Leica M9, and the Sony NEX-5 (with an adapter).
Surprisingly for some, the Sony small inexpensive NEX-5 is giving pretty much the same results in quality even if the lens adapter is slightly reducing the field of view.
You can go and check the samples photos on the original post.
Thanks to Steve for an interesting comparison idea. It is showing that digital quality is improving in a way which is reducing the picture differences between cameras while leaving a lot to compare the other features of the cameras.