Hi! Nick.
My concern with the Solanum species and its poisonous effects arises not only from my gardening experience, as noted, but from reading of the potato's, (virtually the same plant), association with solanine and other toxics.
Check out: -
http://en.wikipedia.org/wiki/Potato

- bottom of the page.

Quote: - " Potatoes contain glycoalkaloids, toxic compounds, of which the most prevalent are solanine and chaconine "
" Solanine is also found in other plants, in particular the deadly nightshade "

My suspicion is that if these componds are down there in the tubers/roots; then there must be a possibility,at least,of some "leakage" into the surrounds. The fact that these substances are toxic to humans, of course, doesn't mean they are necessarily soil pollutants with regard to other species. However!!!!!! Who knows??? Would explain my practical experience.

Regards dave