For the same reason, the ferromagnetic nature of the magstripe, it is possible to read, store and playback the data information on the stripe. You need only few $ for the reader or a simple Square reader. This is an alternative solution to spoof Magnetic stripe cards on Android: http://cosmodro.me/blog/2011/mar/25/rhombus-square-iskewedi/
Hey, cool, it's nice to see my stuff get referenced by someone that isn't me!
Rhombus only _reads_ magnetic stripe data. It won't write it. But yes, you could totally write to a low-coercivity stripe with the right hardware.
Before you run out and get a Square reader, they're all encrypted now and don't return the raw data necessary for Rhombus to work. You can still get unencrypted readers from China on alibaba pretty easily.
I actually read your thing a long while ago too! I took it a step further, and made a small coil and amp, and played the recorded data back through the coil, and placed the coil next to a one-track card reader. It actually worked, which was neat.
I remember requesting a stripe reader when I first saw something about them just spitting out the info, and by the time I got it they were already encrypted.