First question, why would someone?
A long time ago it was possible to run mythtv on debian stable.
Today it’s only possible to run mythtv on sid (unstable) because the mythtv packages moved out of the stable branch a long time ago. No need to build mythtv from source as we nowadays use pre-build mythtv packages from multimedia repository maintained by Marillat: http://www.debian-multimedia.org
For that reaons I run Debian sid on my mythbackend. But everytime packages were updated through apt-get, I was often left with many broken packages which I had to fix manually, leaving my system really instable. (well that’s what sid is all about….) Not to speak of my wife who got angry because her favorite programs were not recorded…