[iiilizium]

"Every sentient being" is literally animals as well, is it not? "birthright" is something understood to be in the same lifetime as that birth.

Sogyal Rinpoche isn't saying that enlightenment is every beings birthright, merely that Buddha Nature is present within all. All sentient life possesses it but the potential at any time to realize enlightenment is vastly different according to individual conditions. They are not the same thing.
Also, is the "good news" about "buddha nature" in the Pali Canon? Could someone point me to a sutta there, please? I've never yet seen it but that doesn't cut any ice within the Mahayana schools. Their Sutras are their authority, so there's no milage in pointing that out to them.

The only way to comprehend it, is within their own frames of reference and then make a decision on the validity of it for ourselves. I tend to sort of equate (perhaps wrongly) Buddha Nature with the Dharmakaya.

http://en.wikipedia.org/wiki/Dharmakāya

Namaste
kris