Our company is soon needing this kind of functionality.
Issues seen from previous comments, but not yet addressed:
-delete's only show the oid (not anything else about the element that was deleted), so how can you create triggers around deletes? You would really need to know more about deletes to trigger from, and doing logical deletes is a time wasting work around
-sharding, and having to worry about multiple oplogs
-optimizations not available (search for 'interesting optimizations' from John Crenshaw)
-oplog format isn't a portable solution (format may change in the future), so its really more of a hack than a real solution
Eliot, do you really want the all the developers out there needed trigger functionality parsing the oplog? If so, fine, I'll write my own version of code to do so. If not, can you me know what would be required to move this forward? My company wouldn't be able to afford funding this alone, but would have funds to put towards this.