Yes, it's a proven morph. It's a naturally occuring dominant/co-dominant gene. Nobody has produced a super form, so this is probably a dominant gene. Sometimes referred to as Sugar or White-Sided morph. Lots of combos have been done with a calico. Here's a good one: Shattered dream (calico x pastel x shatter). Here's a picture of it: http://www.exoticsbynature.com/07tinleyphotos.html
Snake owner since Oct 2008, so yeah, I'm no expert. 0.1.0 pastel royal 1.0.0 spider royal 0.1.0 albino royal 1.0.0 bumblebee royal 1.0.0 yellowbelly royal 0.0.1 wild-type royal 1.0.0 normal western hognose
Perhaps, if it is wild caught or a kid of a wild-caught whose owner doesn't know anything about calicos.
Since it's a dominant trait, people breeding calicos will know if the kids are calicos. Since they fetch a good price, I doubt they'll mistakenly sell them for normals.
Snake owner since Oct 2008, so yeah, I'm no expert. 0.1.0 pastel royal 1.0.0 spider royal 0.1.0 albino royal 1.0.0 bumblebee royal 1.0.0 yellowbelly royal 0.0.1 wild-type royal 1.0.0 normal western hognose