heh heh...we did the same thing except we used company ballons and saran wrap.
I think the best practical joke I ever pulled at work was renaming a conference room right across from my cubicle. All of the conference rooms were named for obscure local trees except this one. So we replaced the name plate and chuckled for three weeks while people walked back and forth and commented. Then it actually started to get used around the building. Since there were, essentially, two bosses....each thought the other had changed it so no one went to correct it for several weeks.