You're right, you've become a good network citizen, and just in time. Cities are starting to establish laws that define leeching off another person's wireless wifi network without permission as an illegal act of theft. How you'd get caught I don't know, but you can imagine that from the other party's point of view, it'd be alarming if you were, say, pirating movies and it was their computer network that was tagged!

Like many other computer systems (think Windows Vista, for example) the Mac OS X system is smart and tries to simplify your life by remembering what you're doing and automate the process subsequently. Sometimes that's a pain. :-)

What happened was that you have a setting in your "Network" System Preferences that tells your computer to remember networks you've joined and prefer them over new ones that it hasn't seen before. Since your neighbor's network is now marked as a "preferred" network, well, you know the problem you're seeing!

I actually had a similar problem in a hotel room during the Consumer Electronics Show recently, where the first night I was there I used the hotel network "Encore - Rooms - Wireless", but then set up a wireless router so my roommate and I could share a single connection.

In the pull-down menu from the wifi icon on the menu bar, I saw this:

The shortcut to get the right spot to change or forget the preferred wireless network can be reached by choosing "Open Network Preferences..." at the bottom of that menu, which takes you here:

Don't worry about what network it shows you as connected to at this point. You want to click on the "Advanced" button:

Click on the "-" button and that wifi network is no longer on the preferred list. Also notice that right below it is the option to "Remember networks this computer has joined": if you want to choose a network each and every time just unselect it.

Done? Just click on the red button on the top left and it'll ask if you want to save the changes:

That's all there is to it. Good luck, and thanks for not stealing bandwidth. :-)