A bit of googling suggests it is called a "Ghulra." It's supposedly like a fingerprint to warforged - an identifying mark that is used among warforged to ID each other similar to names or faces.
I'm trying to find it in a book as I type.
[EDIT]: So far:
4e Eberron Player's guide makes a mention of it, page 33, left column.
Dragon 364, page 28, middle column.
3.5 Player's Guide to Eberron, page 150, first paragraph, no name given.
[EDIT2]: Siberys (Oh, the irony!) scores big with a pair of nice, juicy paragraphs of description.