This announcement came by in February - IBM is now offering something called the Rational Modeling Extension for .NET for pieces of its Rational software development portfolio.  Philosophically, this is the replacement for Rational Rose XDE, the well-regarded UML tool for .NET from IBM, that (I believe) IBM discontinued, as well as Rational Rose Developer for Visual Studio.  The Extension for .NET adds on to Rational Software Modeler or Rational Systems Developer.

IBM says:

The UML 2 modeling capabilities available for use with IBM Rational Modeling Extension for Microsoft .NET software include object diagram support (new in V7) and extended capabilities for modeling activities, composite structures, components, interactions (sequence and communication diagrams) and state machines-in addition provide use-case, class and deployment diagramming capabilities.