Is the Mainframe the Best SOA Platform?

A while back I had a post about a major US bank running a web services performance test and getting spectacular results using multiple zSeries Application Assist Processors (zAAP). James Governor makes an interesting point that using zAAPs on the mainframe can significantly reduce software licensing costs, making it a potentially attractive platform for new SOA-related infrastructure. A specific customer example with Farmers Insurance is cited here. Another post says that mainframe CICS customer are upgrading to the lastest CICS version which is XML- and web service-enabled at a faster pace than seen by CICS in 35 years. The same post claims 25% of z-Series mainframes are now running WebSphere Application Server... thats one in four. This post gives a customer reference from Wachovia (only the 4th largest US bank) which is running WebSphere tools on the mainframe.