Supposedly its not demons, or Tyranids, it's a splash flyer release, which to me at least is way more believe-able that the two previous rumors.
Remember kids, these are just rumors...
Flyers and Deamon Waves
Whether or not the next White Dwarf is Daemons or Flyers doesn't matter much to me. The highlight for me is that its two months of 40k that gets me all excited. Yes, I am drooling a little bit on the next flyer wave, but waiting a month is OK.
So from sources it does look like we will be seeing that elusive flyer wave we have been waiting for so long to see. If you have been following the rumors this week, the rumor mill was saying that it was either this coming month or next month (probably March) that we will get the flyers. I like it especially since it seems that we will be getting an insert or data sheet on the new units. So fingers crossed the Harpy gets some new update from the current codex.