I figured out what went wrong. Or at least what I needed to fix it.
I followed the instructions on this page about attributes:
http://www.red-gate.com/supportcenter/c ... Attributes
I started w/ the first option listed on the page, and added a reference to SmartAssembly.Attributes.dll, and then later used the 2nd option.
Even though the app no longer actually needed the reference to the S/A dll, it was still being referenced. And when deployed, insisted it couldn't run w/out the S/A being in the GAC.
I removed the reference, built, and deployed, and the problem went away.
Still weird that option #1 would cause this though.
Seems #2 would've been listed first.