Edit: All indications point to a rumoured late 2013 release.
I've heard from two sources within GW that they have seen work in progress CAD thingamies for Dark Elf plastics, of which Witch Elves were named specifically. Another kit was mentioned as being Black Guard or Executioners. There was also another plastic kit he didn't recognise and some plastic characters. I'm embarassed to admit I can't find the original email to check exactly what he said. I think there may have been some large plastic critter as well.
EDIT: The other source just mentioned Witch Elf CADs which he wasn't too impressed with.
I've had this corroborated by another rumourmonger who said it fits in with what he heard of Dark Elves coming in 2013 and CAD images meaning they are usually 18-24 months or so away from production.
As always, until pictures are leaked or GW sends out an email treat it as rumour not fact.