Thank you both for your contrasting replies.
I'm assuming that mjhudston is referring to the fact that it doesn't show up on the network map in Windows (or other OS equivalent), and that Valentin means that the router (or network software interface) sees it as a network device. Please correct me if I'm assuming wrong on either. I know that Vista does not see my Kindle at all when it builds the network map, but my router sure knows it's there, allowing me to track its traffic, block it, label it as an intruder, etc. That's what I was referring to.
Thanks again, both of you.