There are two "official" lists of standardized common and scientific names for herp in the US.
The more official list, accepted by the Society for Study of Amphibians and Reptiles (SSAR) and the Herpetologists League is the one by Crother, et al. (http://www.herplit.com/SSAR/circulars/HC29/Crother.html). That list uses one word - milksnake (and also ratsnake, kingsnake, rattlesnake, pinesnake, gophersnake, etc).
The less official list, only really favored by a small group of people is the one published by the Center for North American Herpetology (CNAH - kind of a splinter group off the SSAR) (http://www.cnah.org/nameslist.asp?id=6). This list uses two words (milk snake), but it is criticized for its inconsistency (they propose ratsnake, bullsnake, kingsnake, but also milk snake, pine snake, gopher snake).
Therefore, I say the official name is milksnake as the current SSAR list is making an effort to provide consistency and is therefore a better list.
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Chris Harrison