It's called 'get your own internet connection'.
I'm all for casual 'borrow someones access point to grab a couple kilobytes of traffic data' and whatnot, but using your neighbor for your entire browsing experience is excessive. This is just my opinion, btw.
Another reason to do so is that if your neighbor is wiley, they could set up a proxy that grabs all your traffic, including when you check your email and send your password, and get access to anything you have a non-SSL account with. For example, they could post to all the BBSs that you post to under your account because your password is sent in plaintext when you login.
So, in summary, two reasons to get your own connection. First, you're not just borrowing a cent from the 'Take a penny, leave a penny' tray, you're dumping it into your pocket. Second, you're putting all your browsing up for perusal from your neighbor if they so wish, including passwords and accounts.
In answer to you question, if you have to ask, then no. There are a number of clever ways you could mask your presence, from the social engineering side all to way to the technical side (using spoofed MAC), but like a Jedi making his own lightsaber, this is something you gotta learn yourself.
EDIT: Followup, I'm not trying to 'lay down the law' here. If you choose to continue doing it, it's your choice. I'm just posting my opinion on the subject, and I hope you at least consider my suggestions for the reasons specified in the text.