Duane Merrill over at IBM's Developerworks just posted this good introductory overview Mashups: The new breed of Web app. He covers the various genres (as classified by tag here), the technologies including REST/SOAP, Ajax, RSS, RDF and screen scraping. And perhaps the most interesting part, the "Technical Challenges" section that looks at issues in data integration, data quality, and component model issues. There's also a social challenges section with a quick look at licensing and commercialization issues. Nicholas Carr followed-up on some of these issues in his post Bastard apps. He makes good points about data quality, reliability and consistency issues in what he calls Web 2.0's truthiness problem.