yes, require proprietary SDK..
For example you can easily use:
- UniMag, Mobile MagStripe Reader "..reader that works with various mobile devices.., ...Plugs into audio headphone jack – no cables..."
The SDK is there... in page... http://www.idtechproducts.com/products/mobile-readers/112.html
- another reader with own SDK: http://www.magtek.com/support/software/programming_tools/