When Using the UML Reverse Engineer option in Visual Studio 2003, I ran into the following errors:

Warning W0002:  Couldn't reverse engineer a namespace.  Error number 131.  No such interface supported.
 Element name:  Somenamespace.SomeClass

Number of warnings: 1

...

Reverse engineering failed.
There were warnings during reverse engineering.  Error number 56.

Come to find out what it means is that one of the interfaces SomeClass implements was not in the same project or whereever else it looks (GAC maybe).

Solution:
Put a copy of the interface in the project that you are trying to reverse engineer.  This makes it happy and all is well.