Hi,
Visual Studio 2008 doesn't include support for UML. There is a class diagram, but it is a diagram tailored to visualizing .Net classes in your project, and is not a UML diagram. The good news is that in the next version of Visual Studio we are adding support for some of the UML diagrams - check out Cameron Skinner's blog for more details
(
Skinner's Blog : Visual Studio Team System 2010 Architecture: Prologue is a good starting point).
Thanks,