I am not really qualified to advise you on this, but let me share my gut feelings for all I know

CAM in stbs is meant for interoperability among various DTH providers, so that one can use any stb and insert whatever card they want to (just like mobile handsets and sim cards). However, that doesn't seem to be the case with current DTH services in India. I have never heard of anybody using other stbs for
tata-sky or dish-
tv, like the ones made by
India's best technology for DTH terminals from MCBS or other (Chinese) boxes readily available here. Once I had inquired about such stb's, but all of them claimed that they can support only FTA channels, not paid ones. Apart from compatibility with CAM modules, tata-sky and dish-tv may have other customizations in their stbs as well (like all "active" frills).
I guess Reliance will also follow the same, and support for mpeg4 will be a necessity (of course!)
@yash,
BTW, DVB-S and DVB-T have more fundamental differences other than just thin versus thick cable heads. So, the converter that you mentioned might be having some circuit inside as well, I guess. Is it so?