I think you have to have the NTTA issued tag for parking at Love and DFW, not a TxTag. On the airport web site they only mention the TollTag, not TxTag, for parking, and I also do not recall seeing the TxTag logo at DFW and Love for parking.
http://www.dfwairport.com/parking/tolltag.phpI noticed Wikipedia had a blurb on the TxTag not yet working at Love and DFW.
http://en.wikipedia.org/wiki/TxTagHope that helps clear it up.
[This message has been edited by andy_aggie (edited 6/25/2010 10:18p).]