When I use the neighbor's access point or any other that is not locked, I would treat the connection as my own in terms of abuse. I have WiFi at home but I have it locked based on WEP code and MAC address.
I have the same situation... my mother-in-law if comfortable using dial-up AOHell to send e-mail and sees no need to pay for a faster connection. Lucky for me, her neighbor installed a new D-Link Wireless-N router and left it unsecured.
I stay off of high-bandwidth sites when using that access point. I *could* have viewed YouTube to my heart's content (that wireless-N is *faaaast*), but I figure that some discretion is warranted.
Besides... I don't want my activity to cause the owner to enable the password. If he/she does, it's back to the coffee shop when we next visit.