Some links on Mixture Models
http://en.wikipedia.org/wiki/Mixture_model
(watch out, this is wikipedia, but when I read it, it was very helpful)
http://www.cmh.edu/stats/ask/mixture.asp
http://www.csse.monash.edu.au/~dld/mixture.modelling.page.html
http://www.ceremade.dauphine.fr/%7Exian/mixo.pdf
http://portal.acm.org/citation.cfm?id=507482