Istanbul is for servers
Keep in mind Istanbul is the code name for the server parts...not desktop. The current generation of AMD Opteron servers have been using Socket F since 2005. Since the release they have supported dual core Santa Rosa, QC Barcelona, QC Shanghai, and now 6C Istanbul, all using the same socket architecture. Pretty amazing.
A word about Dunnington. Most people and jounalists forget that Dunnington is just for 4 Socket servers. Intel doesn't have a 6-core product in the 2-socket space. Istanbul should be a 6-core product for 2S, 4S, and 8-socket.
Also, QC Shanghai does a decent job of competing with 6C Dunnington in a lot of workloads and benchmarks. Just take a look at VMware's VMmark for example. Virtualization results based on Shanghai lead the charts. Even the 16-core VMmark scores top the 24-core results (Dunnington).
Not bad using a platform that has been designed and leveraged across 4 processor generations over the last 4 years.