I’m late to the party in writing about firesheep. For those not into web security, it basically means that it has become easy peasy to grab someone’s credentials when they are surfing the web via a non encrypted wireless network–the kind that used to be at a few coffee shops but now are at laudromats and car repair shops.
The upshot: think long and hard about surfing any sites that you sign in to that doesn’t encrypt all traffic thereafter (I’m looking at you, Yahoo! Mail). If you must, consider running blacksheep, change your password regularly, don’t have the website ‘remember you’, and make sure you sign out (which typically invalidates your cookies)–don’t just close the browser.