Head gasket issues are popping up on some of the 05ish Dmax trucks. There's something about the way the gaskets were crimped that changed around late 05 early 06. There's a bunch of information on dieselplace.com about it.
I have an 06 LBZ and am pretty sure I have the later gasket but have been watching this closely. Yours is honestly the first one I've read about that is on a stock truck. All of the others have been modded.
That really sucks... sorry to hear about it. Here's a link to a thread I started asking about the cost to replace. There's some really good info in here:
http://www.dieselplace.com/forum/63-gm-diesel-engines/140-duramax-third-generation-2006-2007-5-lbz-lly/477216-how-much-headgasket-job.html
