Here's a list of IBM-published Redbooks relating to .NET

WebSphere and .Net Interoperability Using Web Services.   366 pages. June 2005. 
DB2 UDB V8.2 on the Windows Environment .  356 pages.  October 2004.
Integrating DB2 Universal Database for iSeries with Microsoft ADO .NET .  282 pages.  April 2005.
Lotus Domino and .NET Coexistence .  ??? pages.  April 2004.
Websphere and .NET Coexistence .  606 pages.  May 2004.
WebSphere MQ Solutions in a Microsoft .NET Environment .  360 pages.  January 2004.

I haven't looked at any of these in detail yet. 

In addition, IBM has published a bunch of other papers and articles on using .NET with pieces of IBM infrastructure:

IBM DB2 Data Access Application Blocks for .NET   April 2005
Creating and Linking LOBs with .Net and DB2 UDB   April 2003
Developing .NET Applications with IBM Rational XDE using C#   May 2004
Websphere and .NET working together   August 2003
ADO.NET and Cloudscape   October 2004

In addition, don't forget the various software IBM delivers:

  • MQ Classes for .NET
  • DB2 - data provider, VS.NET plugins, and .NET stored procedure hosting
  • Informix .NET provider
  • Tivoli Access Manager for .NET
  • Tivoli Monitoring for .NET
  • Rational XDE for .NET

IBM also offers services around .NET:  implementation and training, too.  Maybe IBM is starting to come around on this .NET phenomenon? ;)