How are mashups different from dashboards that have been around for a while?

Mashups are about the assembly of data, so they include data access capabilities and data security features. Mashups also emphasize 'web 2.0' style collaboration, so you'll see mashup sharing/tagging features. Finally, mashups can be delivered to any output/destination, so you can expect the same mashup to be available in a portal, in a dashboard, on a mobile phone, or in a spreadsheet. In short, dashboards are a great destination for mashups; they overlap logically, but not in practical sense.