Check out these links for additional context and resources related to what a Mashup Day is all about:
http://www.mashupguy.com/ , check out the Mashup Lounge at MIX:
http://www.visitmix.com/mixtify/ , and a couple definitions: "Mashup Days engage diverse professionals in a creative environment to envision and create the ‘next big thing’ in meeting customer wants and needs" and
http://en.wikipedia.org/wiki/Mashup_%28web_application_hybrid%29. I also think about Mashups a bit like tira misu, you need to experience it to get it. Remember that scene in Sleepless in Seattle? - Sam: What's tira misu? Jay: You'll find out. Sam: What is it? Jay: You'll see.