I gather it is big in music (mixing a hodgepodge of existing tunes and lyrics together into one song etc.) and now... RSS.
"Mashing up is turning into a meme -- first it was music, then it was Web services.. now it's RSS feeds.
RSSMix (available at http://www.rssmix.com/ ) Allows you to specify any number of RSS feeds to be blended together into one uberfeed. Once you have an uberfeed there are other things you can do with it, but one thing at a time."
Also mentions "Excel blogs" yikes!!