Can the built-in OS X "Location" feature auto select locations based on wireless networks?

Since you mentioned Firesheep, let me put in a plug for Sheepsafe:

Sheepsafe was built to automate the task of switching your network configuration to use a SOCKS proxy whenever you join an untrusted network.

Sheepsafe works by keeping a configuration of known safe wireless networks. When you join an untrusted network, Sheepsafe switches to a network location that has a SOCKS proxy configured and starts a SOCKS proxy by SSH'ing into a remote server, thus protecting your browsing traffic from FireSheep and other snoopers on the local network. When you switch back to a safe network, Sheepsafe switches back to the default, trusted location and shuts down the SOCKS proxy.

There are a number of location switch apps that work along with the Location feature such as NetworkLocation and LocationX. A more recent release you could also try is Airport Location