CICS Transaction Server, sometimes known as CTS and sometimes just called CICS,  added support for WS-AtomicTransaction ,  as of November 2005.   There's a tutorial for setting this up, on the IBM website. Essentially CICS TS can act as a provider or consumer of web services.

Here's an overview of SOAP-for-CICS. (from IBM).  To build apps for this thing, you need WebSphere Studio Enterprise Developer, whoops!  I mean Websphere Developer for zSeries.

If you have questions on how this stuff works, I suggest the CICS listserv.

As far as interop goes, I would guess the basic stuff, theWSDL and SOAP, just plain works, when connecting CTS3.1 to .NET 2.0, or CTS3.1 to WCF 1.0.   I am not sure on the version of the WS-AT spec IBM have implemented (you know it went through several iterations) and I do not have a CICS TS 3.1 region sitting around, ready to test this out on. . . .  So I cannot say for certain that the WS-AT provided in WCF will interoperate with the WS-AT provided in IBM's CTS 3.1.  I will check on this.  if anyone has information on this in practice, I would love to hear it.

Related: see Andy Milligan's recent post on the WS-AT configuration panel in the WinFX CTP.

Cheers,
-Dino

[Addendum - Andy Milligan points out that you can get WS-AT interop to CICS Transaction Server, via WCF and its integration with HIS2004.  Two things:  first, Nice!  I love it when things are easy.  Second: Whoops!  I should have mentioned this.  I like this option because it eliminates the need to upgrade host software, which is always expensive and tricky.  It also eliminates the question of whether the WS-AT on CTS 3.1 is the same version as the WS-AT on WCF, and it eliminates the need to spend $5885 per user on the Websphere developer tool.  Check out Andy's post for more on how this approach works. ]