Shortly before leaving yesterday, with the help of one of my coworkers I updated one of the files in my current build and kicked off a new build. When I got in this morning I loaded it up, and encountered the same error. A quick check determined that the file I'd replaced hadn't actually be compiled, and the object file was still from the old source.
So I did the equivalent of a "make clean," or I at least tired. Net result, the object file in question was still there. Now I've manually removed just that file and kicked off a new build. If this fails, I'm in for the long sequence of deleting everything and getting fresh copies from perforce (about a 1-2 hour process a lot of the time) and then a full build (another 1-2 hours).