Uh-oh
This area of topic is a global nightmare.
I'm from Sweden and we have different laws here than US.
So what might be illegal in your country might be legal in ours.

But for the record I share my findings from then.
I spent half a day (half a year ago) investigating this question mainly asking the Swedish Justice Department
Of course they hadn't heard about WLAN then...

From what I understood from this, the law here is like this -if you do something with the *intent* of doing harm then you can get prosecuted.
Otherwise not, (of course this may be from case to case)

Also from my discussions with the IT-police they then hadn't any clues how to interperate this situation, after I briefed them about what WLAN is...

So here another hypothesis to debate =)
I'm walking along with my laptop turned on with my Cisco Aironet downtown. Suddenly my personal FW alerts me.
A fast check reveals that I'm on someone's net -they gave me a DHCP address -how nice of them .
But their system is also configured to automatic SNMP scan on all devices in their net. And neither I or my FW don't like that...

Q -Who is the intruder now ?

/Daniel