Yes, it is a loophole of sorts - but it requires that you
- Already suspect a user or users of down-voting you
- Assume that nothing else is going on that might affect their reputation
You can strengthen #2 considerably by deleting and undeleting repeatedly - however, this also requires you to be able to predict when the recalc will take place. It isn't actually instantaneous (yes, reputation is kept mostly up-to-date, but the recalc that happens in response to post deletion is queued and executed after the deletion, usually several minutes later). And itthis unpredictability could probably be made less soincreased if necessary - IMHO, simply adding a bit more uncertainty into the system with respect to timing would probably suffice to make this a non-issue. As yoda notes, there are already ways to accumulate evidence of down-voting if you have someone specific in mind.
That said, this does bother me a bit. A dedicated user with a vendetta could confirm his target much more readily now than in the past. We'll want to keep an eye on it.