MiFi cards, or hotspots, normally used to share your 3G Internet connection on a wireless network, are certainly a very useful device for those who need to be connected at any time, no matter where they are.
Unfortunately though, sharing your network connection, aside from providing a useful service to other Internet users, might be a very high risk operation as far as your security, and the privacy of your data, is concerned, as it may lead to sharing some information that should instead be kept strictly private.
For those who living in the United States and who use hotspots manufactured and marketed my big providers such as Verizon and Sprint, the risk lies in a flaw which has been identified by some hackers, and which might lead people with malicious intentions to get to know the GPS coordinates of your position, while you are sitting checking your email, completely unaware.
Of course, this kind of attack requires a certain technical knowledge, but the dangerous part is that it can take place without password or key authentication, while the user has no idea of what is going on right before his eyes. On top of that, it looks like it is possible to download system configuration information, including private information stored in a non encrypted way on your computer.
On the other side of the fence, law enforcement might effectively use this tool to pinpoint dangerous criminals thanks to their Internet connection, follow their movements, gather evidence on their whereabouts and track them down when necessary.
To cut a long story short, if you have something to hide, or if you fear that your private information might end up in the wrong hands, from now on be very careful when you are connected to a wireless network!