While this is not really news, it is a very nice description of a very widespread risk. This issue here is that many websites simply use a serial number in a cookie to keep track of user sessions. The implicit behavior is that if you have the cookie, you are authenticated and logged in. The big problem is that most of these sites are also insecure. With the popularity of insecure WiFi networks, capturing those cookies has become very easy. Once an attacker has the cookie, he can act as you for all purposes on those websites.
The simplest solutions are: enable SSL on the website (if possible), only use WPA secured WiFi, use a VPN, or use Anonymizer with the encrypted surfing option enabled (which effectively makes all websites SSL protected).