Sam2: You may not be aware that many people hold that even if rennet is derived from a dead animal, the cheese made with that meat abstract is kosher. There are shitahs that hold that even if the animal was treff, the cheese would be kosher. Those people who allow cheese made non-vegetarian rennet, will probably hold the “meat” made from animal stem cells is kosher and parve (depending on the nature of the nutrients used to make the “meat”). Those who reject cheese made with non-vegetarian rennet will probably hold that the “meat” is kosher only if the initial cells were from a properly kashered animal.
Since this is still really more science fiction than anything else, its a bit early to worry about the matter.