See the proposal at <http://groups.google.com/group/mozilla.dev.platform/browse_thread/thread/4b468b2f00c5a18a>. Summary is:
1) Abs pos frames are marked as reflow roots.
2) Deletion of an abs pos frame just updates overflow areas up the tree (hence
the dependency on bug 524925).
3) Insertion of an abs pos frame just updates overflow areas up the tree
(though this could run into issues with auto-offset stuff; in those
situations we may need to reflow the parent to get the placeholder placed
4) When reflowing an abs pos frame, update overflow areas up the tree.
We would need to make it ok for reflow roots to change size/position as part of reflow....
Oh, and I took this for lack of anyone else to foist it on, but people are very welcome to steal this!