Here's a promising note for those who enjoyed the Scourge invasion world event a while back. When a player moans on the forums that he thought it was a yearly thing (and that it might never return), world designer Kisirani says not to jump to conclusions: we may be using the Necrotic runes from the event very soon in the future. Rumors have been swirling about a world event for a long time now, and if you've been paying attention to the spoilers (that link is a spoiler, of course), you know that things are going to hit the fan very shortly here.
Though for all of the hints and leaks and files found in the beta, we haven't actually seen an event at work, and it's not completely clear just what will happen. Of course, something has to happen to Naxx for it to move north to Northrend, and we've speculated all along that, without being too spoilery, Arthas would do something to drive the Horde and Alliance to seek him out, and that's exactly what the hints are pointing at. But as for when this event will go down, and what, if any, rewards might come out of it, from Necrotic Runes or otherwise, we don't know for sure. As much as players have learned about how Wrath might kick off, Blizzard is still holding their cards close on this one.