Why are mashups via ESRI important? It will allow cartographers to create custom maps with our own look-and-feel, with thematic data, and still be fully enabled with the “mashup” web 2.0 mentality using APIs similar to those from Google and Yahoo.
The earlier post mentions a REST javascript API for interfacing with the Arc Server to create mashups. That’s true in the first release, but a followup release will allow Flash / Flex integration as well.
Read more from flex888.com and see an example.
More info on the REST api from mandownnz.com.