i do it quite a bit, i've picked up over 2000 networks, but i live in los angeles, around 50% of them use WEP i'd say, of course just because they don't use WEP doesn't mean they are open and i don't usually check any further. The i upload the data to various websites.
Its actually not illegal to wardrive, remember you are transmitting your signal all over the place, even to my house, whats illegal would be stealing bandwidth, data or circumventing lock outs, anything beyond this is basically the lawyers and judges arguing over whos wrong to do what, and intent. My laptops OS automatically scans for highest available signal and connects to it, sometimes i don't even realise it connected to the wrong AP when i am out on site.
Its a bit like tuning into a private radio station that didnt encrypt, but they can always once you realised you werent supposed to be connected to it, why didnt you disconnect ?
sailor420: netstumbler might support cisco by NDIS support.
I'm writing my own tool thats like netstumbler, but with a lot more features for both wardriving and network admins, i was going to do a request for features post but i want to get some more ground work into, i'm still working on the core itself, when its ready i'll release it on tfp for whomever wants it
I use orinoccos and prism 2 cards, with one of those little kingston wifi detectors for quick tests (which is pretty crappy)
The best AP i found so far is a wide open 1.5MB each way in the middle of griffith park, that so far i haven't found anyone else using, so i can go out and enjoy the park and still work etc, i stopped to grab a can of coke from the machine and sat with my laptop to fiddle around for a bit, next thing yahoo IM pops into life.
I think warchalking can be a good idea so long as you don't screw anything up, it'll make people rethink their wireless security, of course i doubt 'the man' agrees

most attempts to justify anything like this by any types of hackers ususally fails since you have to argue against something thats black and white to them.