Introduce a mix of the flat/threaded mode by having a mode where ALL replies to a post are showed in flat mode. All the subsequent replies to the replies should not be shown.
This will have the effect of presenting all the answers to a given post that belong to the same level so to speak, thus producing a logically consistent subview of any given thread-tree.
For example, consider this scenario with posts on various levels:
1 (starting post)
--2
---3
----4
----4
----4
---3
---3
--2
--2
Now, what I suggest is that when you choose post 1, all the 2's are shown as well (in flat mode that is- perhaps a little indented). Likewise, when you choose post 2, all the level 3 posts are shown as well...and so on.
I think this (for most boards and topics) will produce a coherent image of the post and its answer(s), while still maintaining the tidyness of the threaded mode.
Of course, all the displayed post should be clearly marked in the threaded wiew below.
Tim