I've added a rev_deleted field to the revision table in CVS HEAD. If running a wiki on 1.5 CVS currently, run update.php or manually source maintenance/archives/patch-rev_deleted.sql to update the table.
This will be part of changes to the deletion system, not yet fully implemented:
* Deleted revisions will remain listed in the contributions history (though greyed out and in strike-through), making it easier to track serial offenders.
* Deleted revisions will be listed in the regular page history (again, grayed out and in strike-through). In cases where individual revisions have been struck from a page for eg copyright reasons this will make it easier to see where portions were removed.
* The actual text won't have to be shuffled into an archive table; simply updating a 1-byte field in the revision records ought to be faster, and this will allow deletion again on pages which have been block-compressed. (Block compression combines multiple revisions' text into a single compressed record; since this interacts poorly with the current deletion/undeletion system, deletion is currently disabled for any page which has been block-compressed.) Deleted text could be pruned at leisure by a batch or background process if necessary.
* For admins, the diff and view source tools will remain available on deleted pages, which the current Special:Undelete viewer doesn't provide. This can aid in analysis of a vandalism spree.
The deletion/undeletion system has not yet been updated to actually use this field, nor has limitation of viewing of such revisions to admins been implemented. (Remember the purpose of _deletion_ is to make materials inaccessible to the public, especially if for legal reasons, so non-admins will still not be able to look at the actual deleted revision text.)
Once finished there will probably be a slight change to the page table as well, to mark pages which have no non-deleted revisions.
-- brion vibber (brion @ pobox.com)