Sounds like MashUp is yet another well-tooled layer. Layers get born (or just get named) when the popular layer de-jour fails to be the golden solution and is becoming silo-ed. Mashups, as I see it, are subject to the same trends and, possibly, the same fate of being over-ed by yet another layer. An environment with a reasonable SOA implementation may claim MashUP to be just another massive SOA service.

Many analysts (like Forrester, Gartner, IDC) position mashups and SOA as very close cousins. That said, few of the major SOA providers provide useful mashup capabilities. Logically, I guess you could merge the mashup layer right into the SOA layer. But then you'd be missing the point that mashups DO NOT require a SOA. Remmeber, in most organizations the SOA initiative is far from complete, and a mashup initiative would NOT need to wait on the SOA effort to yield returns to the business.