An ad hoc network typically is not a network at all, in the sense that the OP is thinking. It's probably just somebody else's PC.

Connecting to a random available unencrypted Infrastructure network is one thing. I happen to believe that if I see an open access point/infrastructure network, that is radiating an invitation to use it. I do take extreme care, raise my firewall protection levels to "paranoia", only use SSL or TLS for POP/IMAP mail, always use https for webmail, and/or use an IPSec VPN to trusted systems where available. Although the "correctness" and ethics of this may be open to question, technically it's legal in most places, and I believe that it's ethical - there are plenty of open access points that way on purpose.

But connecting to a random Ad Hoc network is unethical snooping around, IMHO. As well as stupidly risky. Given that it's 90% certain to be somebody else's PC (unless it's that suspicious "Free Internet Access" ad hoc network I keep seeing too), it's like you're rummaging through their files while they're away from their seat. In most cases you can't even get to their files, but if they were dumb enough to leave their PC open as an ad hoc network, they may also have been dumb enough to turn on file sharing without a password.

It's not some special secret part of the internet with stuff for you to discover.