LLY doesnt stand for anything specific, its just the RPO (regular production order) code that means "Duramax diesel engine". Just in the same way that AN3 means power front bucket seats, UC6 means 6 disc in dash cd changer radio and U2K means satellite radio...I am 99% positive there is no rhyme or reason to lableling RPO's.
The Duramax is technically rated by GM/Isuzu for a 200k mile TBO (airplane speak for Time Before Overhaul). Although that by no means says "the engine will blow up at 200001 miles". With good maintinence and care, many people have made their Duramax's go much further. I forget the guys name but there is a member here who has gotten something crazy like 500 or 600k miles on his Dmax's before he sold them (they were working fine when he sold them)!!! It seems he drives the around the world becuase he has a 2005 that already has ~120k miles! Time will tell what the general life is with these engines, because remmeber, they only came out in 2001...
---Ben