When you connect to a public WiFi network, that does not allow people into your computer; to do that, you would need to turn on file sharing. As long as file sharing isn't on, you're not going to let anyone else in.
So the odds of someone "hacking in" to your computer are pretty much nonexistent as long as you're not running file sharing or some other sharing service.
However, that doesn't mean that public networks are completely safe. There are a number of other things you should be aware of.
The first is that people, especially in airports, will set up bogus wi-fi networks with names like "Free internet access".
There are two kinds of Wi-Fi network: regular networks, where your computer attaches to a wireless router, and peer-to-peer or "ad-hoc" networks, where your computer attaches to another computer that is attached to the Internet. When you use an ad-hoc network, yeverything you do passes through the other computer.
So the way the scam works is that the scammer will take his laptop into an airport and pay for access to the airport's wireless. He will then set up an ad-hoc network and name it something like "free internet access." Everyone who connects to him gets Internet access through his laptop, and he uses his laptop to record everything that passes through. Ever Web site password, every email, every mail password, everything goes through his computer and gets recorded.
Your Mac will clearly show you the difference between a regular and ad-hoc network. When you click on the airport symbol, you will see a list of regular wi-fi networks. Then, under a column heading labeled "Computer to Computer Networks," you will see a list of ad-hoc networks. Never choose an ad-hoc network in a public place; it's likely a scam.
The other thing to be aware of is that when you connect to a wireless network that has no password, all the things you send and receive are being transmitted by radio over the air. Anyone who is running a wireless packet sniffer program can see what you see.
This is not true for secure Web sites (Web sites you connect to using https:// rather than http://) but it is true of anything that is not secure, including email (if you connect to your email servers using non-secured connections), FTP, and ordinary non-secure Web sites. So if you are using a public network that does not have a password, you should be aware that it is at least theoretically possible for someone to eavesdrop on you.
