I'm sure Gary didn't really mean that it is in the body cavity, I think he just misspoke.
Although it isn't really expensive, it is not cheap, either. Depending upon how you buy them, tags can cost $8-10 each or more. For all colubrids and some smaller boids, you'd have to raise the offspring to a size suitable for tagging. Therefore, this will only catch on where there is a lot of interest and enough dollar value to the animals to make it worthwhile. For many species, pattern ID is just as reliable, and cheaper.
Also, there is the cost of the implanting kit (not much) and the reader (several hundred dollars?).
I'm quite sure I'll never PIT tag baby corns! Maybe if I was breeding some rare python... I think I'll stick to tagging only what I need to for MNR purposes.
Maybe I'll offer a new product, though- Jeff's PIT tagging services!