Just as an additional to this topic, you may also need to have the network adapter on your laptop (i.e. the AirPort card) approved to connect to the wireless router of the network that you are attempting to connect to. This doesn't happen at coffee bars and other public networks but I've got a few friends that set their routers to only allow connections from "approved" adapters so as to prevent anyone from jumping on their network (WEP encryption, for example, isn't terribly secure so they're taking no chances). In order to get your laptop "approved" you'll need to provide the administrator of the network with the MAC address of your AirPort Card (System Preferences > Network > Configure... AirPort - the AirPort ID is the MAC address).
This isn't all that usual but can be a reason why you can't connect to a network that your laptop can "see".