There's an interesting Q&A in the CNET forums about whether one is allowed to use a wireless (Wi-Fi) connection that's not one's own. It's asked in terms of one's home, but this would apply to a business, too. What's to prevent a business next door from tapping into your wireless network if it's not protected, or you into another business's?
The basic answer seems to be, aside from the moral issue: Be careful about security (hacking, viruses, and so on). There's another implication for business, though: If someone causes mischief through your network, it's your IP address that will get pointed to as the offender.