On a first quick reading, your post implies that you are using someone else's network routinely "I am using it now..."

A more careful reading shows that you are simply experimenting with one of these "loose" networks by making a test post or two on it, while your real concern is whether or not your own network will be open to such infiltration.

Sorry about that.

The advice on encryption and restricting the network to specific users still applies.

I get a bit nervous about these "open" networks at coffee shops, airports, and the like. Obviously there's no encryption between the computers and the network. How safe are these from someone using the network to punch into your machine?