So, I updated my dll, but the projects that had called it weren’t recognizing the change.
Perplexed, I tried a bunch of stuff and finally decided to increase the Assembly version, which led to a web control working properly, but not my web parts.
Turns out the correct solution was to recompile the webparts–but first I removed the reference to the shared dll, re-added it as Local Only = False (I’d added them as true earlier.) and re-deployed.
http://msdn.microsoft.com/en-us/library/t1zz5y8c(VS.80).aspx is something I should have read days ago.