http://www.cio.com/article/595304/Prison_Labor_Outsourcing_s...
http://newworkerfeatures.blogspot.com/2010/02/us-prison-labo...
Paying someone who you have in complete physical bondage under $1 an hour is slave labour as far as I'm concerned, even if it is nominally voluntary. At best its unconscionably exploitative. Add to that the fact that the US govt. is legally bound to prefer prison labour over any other for the production of body armour, weapons etc. for the security agencies.
EDIT: Here's another fun stat: america has more black people in prison than in college: http://news.bbc.co.uk/1/hi/world/americas/2223709.stm
Although we in the UK are definitely doing pretty well at emulating uncle sam: http://news.bbc.co.uk/1/shared/spl/hi/pop_ups/06/uk_prisons_...