Looks like my vim undofiles only track the last 500KB or so of changes. At the bottom of my gundo panel it goes from edit revision 1774 to 0 (the total number of revisions to the file is 2776). While this indicates the entire history may exceed 1MB, that is very acceptable since this is several days' worth of work (which, still not being complete, couldn't be inserted into the revision control system per my company's code check-in policies).
Is there a way to make it keep more history in vim's undotree? Not that it'll be particularly easy or fun to wade through it all, but not having it throw it away would be nice.