How much info and how much access you can get from a wireless network depends on how much effort the IT people that set it up put into securing it, as well as the rest of the physical LANs setup (as has already been mentioned).

There are several easy, built in, ways to keep the casual “war driver” or park bench surfer out of your wireless network, yet so few network admins take the time to even attempt to enable them.

An example of one of the better security measures that hasn't been mentioned in this thread yet is MAC address filtering.

MAC address filtering allows you maintain a list of NIC card MAC addresses that are approved for accessing the wireless network. In theory, any packets sent from an unapproved MAC address are dropped and not allowed access to the network.

While filtering MAC address access on your wireless network may become pretty labor intensive (you may have to manually maintain a list of your companies NIC card Mac addresses) if you have a large number of computers, I believe it's well worth it.

And yes, MAC addresses can be spoofed so this isn't a fool proof, but MAC address filtering can add one more layer of deterrence when combined with WEP, disabling SSIDs, and changing default router/access point passwords.