Okay, so I'm in object mode, there's a circle around the cursor - exactly like what would be there if circle select were active, but it's not active. Single click on things does nothing, I need to double-click to select anything. And hitting C simply enables the normal circle select. I have no idea how to turn it off, and it's unspeakably annoying this far into a project! Any tips/suggestions would be repayed tenfold in the style of a poem about how great the helpful person is.
BTW - it only applies to this project, none other. So I think I just hit some key by mistake - Thanks.