Nested conversations have been discussed at length on several occasions on the forum. The documentation states:
"A conversation may be thought of as a continuable state. Nested conversations allow the application to capture a consistent continuable state at various points in a user interaction, thus insuring truly correct behavior in the face of backbuttoning and workspace management."
This gives the impression of use of nested conversations to achieve a continuation server approach. This approach is possible and is described at the forum reference, but it seems that most developers use nested conversations for sub-flows ending the conversation at the end of the sub-flow. This does not allow back-buttoning to the sub-flow (which seems to be in conflict with the documentation above).
(I will add a link to an article describing the continuation approach in a few weeks, perhaps it could serve as a patch)