Autumn fern is tough and evergreen but you will still have to clean up some of the magnolia leaves or treat them as mulch? You will want to make sure that whatever you plant under the magnolia gets enough moisture

How about hellebore or liriope? You'll have to blow the leaves off every year though.

I like Walter's idea of Hellebores, you could plant hellebores and Autumn fern, a combination I grow that is very satisfying.

And how about heuchera? The Heuchera villosa "Autumn Bride" I have makes a pretty substantial plant.

@ Douglas, I love Heuchera but I find they don't like to dry out at all so I think it might be a tough spot under a magnolia, what's your experience with this?

macho fern will really spread but might get too cold in the winter. Asparagus fern would do really well and it winters better.

@Erica: Mine are in very well-drained soil, get morning sun only and are in a place I absolutely never water. I've never planted them directly under a tree (although they are under a doublefile viburnum) so I can't say with certainty, but I've found the villosas to be tougher than other heucheras.

thanks Douglas. @ Cynthia B, I don't think Asparagus fern is a good choice for Zone 7 (as far as overwintering).