Something I would like to know as well.
My 8 month old CKCS does the same, she would find her boyfriend (a teddy she has had since 8 weeks) and each night would dry hump it.
Now this has developed into humping anything, she has taught herself to do it with just the carpet and her paws.
I just distract her when she looks like she is about to start! Yet I dont know if this is going to teach her that my carpet is not her boyfriend.