Originally posted by pianojoe
No, they can't.
Yes, they can. WiFi Protected Access (WPA) hasn't been cracked yet, but the lower-level WEP (including 128-bit,) has been cracked. There are UNIX-based (and, I assume, although I haven't played with them, OS-X-based,) programs that can 'listen' to a WiFi session for less than 5 minutes (even if it's closed, WEP-protected, password protected, and not broadcasting its SSID,) and find out all they need to know to allow you to connect.
I know, because someone did this to my own network. My next door neighbor cracked my 802.11b network (running on a Linksys router; I have since upgraded to an AirPort Extreme base station,) and printed to my shared printer "GOTCHA!"
We have since decided to open up both of our networks to each other (two other neighbors within a block have joined us,) so we can all access the internet from anywhere on our end of the block (we even have WiFi going a little ways into a nearby park.) Our individual computers are protected, but for at least two of us, you could just drive by and start printing pr0n on our printers if you felt like it. (No, I'm not going to tell you where I live.)
Hopefully within a year we'll have a 5-block-radius area with continuous free WiFi coverage. Our cable and DSL providers will dislike us, but oh well. Our long-term plan is to get a T-1 or better coming into a 'neutral' area (like the non-denominational church in the middle of our neighborhood,) and use Wireless repeaters (like APEx has built-in) to give the whole neighborhood high-speed internet wirelessly.