Apparently the "Allied Collation Marking"
http://www.scribd.com/doc/58684090/%E2%84%967502-M1A1-A2-Abrams
Page 14 of the doc, actual page 13. 2nd paragraph down on the right.
You'd think it would be possible with GPS and computer fire control, friendlies could be easily ID'd and guns locked out to prevent firing on them.
Edit: I think we talked about that before. You mentioned security issues if I remember correctly.