This is the first article I've seen on the new 3.0L Duramax I-6: LINK

The basics:
From what we experienced in our first drive of a Silverado with this engine, the inline-six Duramax, which makes more torque than the Silverado's available 6.2L V-8 and is backed by the same 10-speed automatic as found in the Camaro ZL1, is a sweet package for those who want a nice balance of towing muscle and fuel economy in a 1/2-ton pickup. It's quick, smooth, and quiet.

Other than the name, the 3.0L inline-six has little in common between the 2.8L I-4 Duramax found in the Colorado/Canyon and the 6.6L V-8 Duramax under the hood of the 2500/3500 HDs.
Pricing:
The Duramax package is priced identically to the 6.2L V-8 as a $2,495 premium over a 5.3L V-8 model, or $3,890 over a 2.7L turbo model.
It sounds like it will be a competitive option in the Duramax lineup.