In many cases, "am I safe" is equivalent to asking "would I trust the environment if I were plugged in directly to the Internet at home (without a router)." You're just as easily attacked from there as from an airport hotspot or wireless network. That's why software firewalls are considered standard for the course now, and anti-virus a good idea.

If you've painstakingly configured your laptop as an ironclad bunker that communicates to the outside world on an encrypted closed circuit connection, I'd say you're pretty safe. :P

Connecting to an untrusted ad-hoc network is no more insecure than connecting to an untrusted infrastructure node.