This story from the Associated Press is good reason for anyone who uses a public computer regularly to take notice. It notes the recent beta release of the Google Desktop Search program, and the fact that on shared computers it has the potential to expose information from one user on a shared computer to another user.
The point is not to bash Google's product, which by the reactions I've seen is quite powerful and cool, but to highlight the fact that public computers and internet terminals are not trusted systems. As such you should never use them to access any sensitive information, such as corporate email, or otherwise transmit sensitive data, including online purchases using a credit card.
The key here is awareness...you should always be thinking about trust, particularly if you are not in control of the machine or network you're using.