Those are the conclusions of researchers from Pennsylvania State University and North Carolina State University, who . They didn't find anything malicious, but a surprising number of the programs used unique identifiers such as the phone's IMEI (International Mobile Equipment Identity) number -- sometimes without obtaining permission to do so from the user.

One concern is that these unique identifiers could be linked to Android users in databases, essentially providing a stealthy way to track what mobile phone users are doing online, similar to the tracking cookies stored by Web browsers. Unlike a tracking cookie, a mobile phone's IMEI cannot be deleted.

The research follows up on work done by some of the same researchers who

Researchers are only now beginning to put together a picture of what's going on beneath the surface with these mobile phone apps, said William Enck, an assistant professor with North Carolina State University and one of the authors of the study. "I think people are starting to become more aware of this, but I don't think there is widespread understanding of what the implications are," he said.

"The paper really expands our understanding of what applications under Android really are doing.... and what they are doing with our data," said Lee Tien, a staff attorney with the Electronic Frontier Foundation.