Over time chemicals leach out of the plastic, especially after being cleaned with strong chemicals (bleach). The largest effect seems to be on developing children, but it is linked to some adult problems.

http://www.ens-newswire.com/ens/jun2006/2006-06-02-03.asp

http://envirocancer.cornell.edu/faq/answer.cfm?use.id=57


There's a couple, I'm sure you can find more articles out there.