Evidence for dark energy equation of state paramater w = -1

[Mosis]

I'm having trouble finding an article or paper on the observational evidence for dark energy being a cosmological constant. I'm looking at this Living Review by Sean Carroll

http://relativity.livingreviews.org/Articles/lrr-2001-1/

and he only describes how supernova data constrain the energy density of DE vs. matter already assuming w = -1.

How confident are we that w = -1, and which observations suggest this explicitly?

 

[Amanheis]

http://arxiv.org/abs/0803.0547

Says with combined measurements of Supernovae and Baryon Acoustic Oscillations, assuming constant equation of state: -1.14 < w < -0.88 (table 2)

 

[Chalnoth]

There's not really a way to find evidence that w = -1. Rather, we have failed to find evidence that the equation of state parameter is sufficiently different. And, as Amanheis properly points out, the experimental constraints thus far are pretty loose.