The current issue of New Scientist has a good story by Paul Marks on mashup security entitled "Mashup websites are a hacker's dream come true". This is an interesting topic that hasn't received a lot of attention yet, but will once a few headline-grabbing security breaches occur via mashups.
It covers a variety of issues in security, privacy and identity -- many of which were discussed earlier this year in a MashupCamp session lead by Hart Rossman, chief security technologist for SAIC. Hart's quoted in this article along with myself, Bret Taylor from Google Maps, and Ben Metcalfe from backstage.bbc.co.uk.
On the privacy front, the story cites one of my favorite mashups (or proof-of-concept of one), Banned Books from Tom Owad.