The New York Times ran an article on the dangers of using public Internet access.
While it is hard to say how likely it is that someone is lurking on a public network, many public networks do not have adequate security.
Last fall, InfoWorld magazine published an article about a security researcher who managed to collect more than 100 passwords, per stay, at hotels with lax security (about half the hotels she tested).
Gathering reliable statistics about security breaches is notoriously difficult, since companies are reluctant to reveal this information. Still, the most recent computer crime and security survey, conducted annually by the Computer Security Institute with the Federal Burea Description of Investigation, found that the average loss from computer security incidents in 2005 was $167,713 per respondent (based on 313 companies and organizations that answered the question).
As Jim Louderback, editor of PC Magazine, noted, the statistics may not matter given the problems one data breach can cause.
The article mostly covers wireless computing. You can read the full article here.