Firesheep allows anyone with a Firefox browser to hijack the sessions of anyone on the same Wi-Fi network using a few dozen popular content, commerce, and social-networking sites by snarfing cookies that pass in the clear.
A virtual private network connection that encrypts all data is the most-secure form of protection, but most people don't buy such service, which retails for $40 to $70 a month. Beyond that, Secure Socket Layer protection for email and HTTPS Everywhere for web browsing are helpful.
If a user wants security, and is willing to pay $40 to $70 a month, some of us might say "why bother?" with Wi-Fi and just buy a mobile broadband service. All the other tools to protect email and web browsing are available no matter what the connection, and a mobile broadband connection is arguably more secure than any public Wi-Fi connection is.