I did my first attempt at a silicone mold for a christening cake last weekend. It was a product called Siligum, and it came as two pastes that you mix together in equal parts ... sounds like one similar to IHATEFONDANT's post above. It was extremely easy to use to make the mold, but Lord was it difficult to use ...in my case anyway! The trouble with the mold I made, was it was of two holding hands, and in order to get all the fingers out in tact, you really needed to demold in opposite directions. I would get one hand out perfectly, then the other one would be missing a finger, or vis versa. When the silicone was set, it was only partially flexible, actually it was quite rigid now I think about it!
Here is the resulting cast in Mexican Paste .... not sure if I'll agree to do one again!!
PS it was quite expensive too .... £15 for I think 100g - no idea what that's in $$