So it's called Sidejacking.
And everyone (in techie world, at least) it talking about it. And everyone should be talking about it - because the ramifications are huge. As one blog puts it:
"Using free Wi-Fi is essentially the same thing as leaving your house with all the doors and windows open – But this takes it one step further: This posts a big sign up as soon as you leave, to anyone who happens to be looking, that says “I’M NOT HOME NOW, HERE’S WHERE ALL MY SECRET STUFF IS, AND HERE’S A MAP TO FIND IT. FEEL FREE TO TAKE, USE, OR DESTROY WHATEVER YOU WANT.” (http://shankman.com/why-its-time-to-say-goodbye-to-free-wi-fi-part-two/)
It's not really the fault of the free Wi-Fi, it's the sites that you access over the connection, and those cookies. And it is all about what you put online, too.
There is a (kind of) fix. Use Firefox as your browser with an extension called ForceTLS, which makes your connection use an "https" - but only if the website lets it. Mozilla (the makers of Firefox) have also said some relevant things in the last day or so. Plus Firefox 4 is a step in the right direction too. This article also posts links to the "fixes":
http://blog.mozilla.com/security/2010/10/27/cooling-down-the-firesheep/
Be informed.