Currently, backing out an integration does not un-mark the source revisions as integrated in the destination. This means that any future attempt to integrate those revisions must be forced, with all of the attendant risk that entails; there is currently no way to actually "undo" an integration, such that P4 will behave as though it never happened. This has caused repeated problems at my company.
A "Back out" or "Revert" of an integration should cause the previously integrated source revisions to be considered "non-integrated" with respect to any target revisions on or after the BackOut/Revert revision.
It seems like this will require at least one "deep" change to the P4 revision data model, to allow a revision to specify that it "un-integrates" content from a previously integrated source. The "Revision Graph" tool will need some way to denote this (perhaps a hollow arrowhead in a new color?)