I did try this out. Luckily, the dry ice was very fine, almost like crushed ice. Heat transfer wasn't really a big problem since I was stirring it by hand (it was at work so my resources were rather limited). In fact, the major problem was that it became too hard to scrape the frozen ice cream off the sides and incorporate it well. I eventually ended up with a bowl of really hard ice cream with a soft centre. The texture evened out nicely after I let it sit at room temperature for awhile. Since I was stirring the whole time, the consistency was rather good, even in the solid parts. All in all, it works well but could be improved with an electric mixer. It was a lot more work than liquid nitrogen though. As for mixing it in, I wouldn't be so sure it's pathogen free. bacteria are hardy little buggers, we've had e coli survive the -80 without glycerol and fungal spores are probably still viable. I'm still mostly worried about chemical contamination though, I wonder how it's stored. Does transpiration do any good? Doesn't the contamination just keep settling down onto the dry ice under it?