Actually I've looked into it since posting as it sounded really interesting. The cardiac arrest theory seems to have been proposed in 1994 (Hardy, D.L., Sr. 1994. A re-evaluation of Suffocation as the Cause of Death During Constriction by Snakes. Herpetological Review).
A 2000 paper on constriction in Gopher and Kingsnakes concludes that these snakes may possibly exert enough force on mice to induce cardiac arrest as well as causing suffocation, but that research wasn't conclusive and the paper stated that further work was needed to confirm the hypothesis. (Moon, B.R., 2000. The mechanics and muscular control of constriction in gopher snakes (Pituophis melanoleucus) and a king snake(Lampropeltis getula). Journal of Zoology).
It doesn't seem that there's been further publication on the subject, so at present it remains a strong theory. Very interesting one though - let's hope there's some research funding for more study.