I've been wearing Arenalines, mainly LT's, for about 18 monsths. Since I couldn't see exactly what the difference was, I wrote to Smartwool and just got this reply from consumer relations:
"There are a few differences in the Adrenaline and PhD socks.
The PhD sock offers many features that the Adrenaline doesn’t. First the PhD offers WOW Technology. WOW Technology adds additional layers of 100% SmartWool Duroyarn in the high Impact Zones. This gives the PhD sock line up to 30% more durability. Next the PhD offer the new 4 degree fit system. The fit system includes the Ankle Band, Arch Band, and the Upper and Lower instep Band. This makes a better fitting sock."