Dino has a post on the potential for WCF to IBM Mainframe CICS interop. The cross-vendor support for common specifications is of course a key benefit of web services and this release seems like a good step forward for some critical line-of-business processes. I hope that in time the interop between the two systems will be glitch free.
I would like to offer an alternative approach for customers using the Transaction Integrator from Microsoft Host Integration Server 2004. If you have the transaction integrator configured today to provide transactional access to your host systems, the COM+ Integration features of the Windows Communication Foundation can work with your transaction integrator components to give a similarly rich web service capability.
With about 5 minutes of configuration you can add WS-AtomicTransaction support for your host system without any additional host footprint and you'll get great interop for WCF and other web service clients.
I am interested in hearing from customers who would like to prototype or deploy this. With the WinFX Beta 2 release out there, everything should be in place to make this happen.