i would suppose acidic to neutral, based on the good performance of mine. assume you are referring to d. odora.
plant them high, protect from harsh mid-to-late day sun, mulch and don't move them. they do not like wet feet but also resent drought conditions. water when needed during dry spells. this has worked well for me.

Actually, a number of Daphnes grow on thin, limestone soil in the Mediterranean and Caucasus regions, while others, like Jeff said, are happier in neutral to slightly acid conditions. What kind of Daphne are you growing or planning on? Robin White has a new book out on Daphnes, available online or perhaps at a local store with a really good gardening section.

The most important thing with Daphnes, from what I've read, is excellent drainage is an absolute necessity, so adding sharp sand, gravel, chicken grit and/or perlite when planting is a good idea.

Hope this helps!

Jim