Version:           1.5.8 (using KDE 3.5.9)
Installed from:    Mandriva RPMs

It would good to support hierarchically linked diagrams in Umbrello. The hierarchy provides a way to structure the project and aids organisation.
I have used Jude which provides this capability and it is incredibly useful.

It is best described with a (contrived) example:

The main Use-case diagram has a "top-level" view of the use-cases which may include Actors "User" and "Admin" as well as Use cases "Use System" and "Admin System".

From the "Admin System" use case in the tree, you can create a sub-diagram of type use-case which could then be called "Use Case - Admin System" which then contained more details use-cases such as "Add User", "Delete User", and so on.

From this, the individual use cases could also have sequence/activity/state diagrams added beneath them.

This gives you a structure as follows (object type in brackets):

Use Cases - Top-Level (Use Case Diagram)
  |- User (Actor)
  |- Admin (Actor)
  |- Use System (Use Case)
  |- Admin System (Use Case)
  |    |- Use Cases - Admin System (Use Case Diagram)
  |    |   |- Add User (Use case)
  |    |   |   |- Sequence - Add User (Sequence Diagram)
  |    |   |   |- Activity - Add User (Activity Diagram)
  |    |   |- Delete User (Use Case)
  |    |   |   |- Sequence - Delete User (Sequence Diagram)

etc.

This provides a structured approach to UML development on larger projects and allows easy visualisation of the relationships as well as easy navigation of the diagrams.