A Google search for "Obama sign stolen" shows that thefts of Obama yard signs are occurring all over the place--Midland, TX; Staunton, VA; Springfield, MO; Ivins and St. George, UT; Sartell, MN; Upper Arlington, OH; and so on. A Google search for "McCain sign stolen" shows allegations about McCain stealing a prisoner of war story, Cindy McCain stealing a recipe, and stories of thefts of Obama yard signs--but no reports of stolen McCain signs.
I suppose either our sign was stolen by an unethical Obama supporter for their own use (in which case the stolen sign should be popping up elsewhere), or by an unethical McCain supporter who has no respect for freedom of speech or private property. I suspect it's probably the latter.
UPDATE (November 5, 2008): Here's a story about a university instructor who wrote about his stealing a McCain/Palin sign in Minnesota--he has resigned his visiting professorship at St. Olaf College as a result. Philip Busse is described in the article as a journalist and political activist from Portland, Oregon.