There was an odd announcement on the WAB Forum last year (the UK forum, not the Yahoo group) wherein it was stated that "Great War" had been cancelled. However, a few weeks later, somebody spoke directly to Rick Priestly, and some of the Forgeworld guys, who assured him that it had not been cancelled, and that "Over The Top" was coming soon. The author of the original "Great War" rules has remained mum on the subject.
The current thinking seems to be that the main focus of GW Historical was on getting WAB 2.0 ready, and following that, the other supplements would be released. However, this has been said before, and it's worth adding to the dilemma two important factors:
1) The last few GW Historical supplements (Showdown springs to mind here) were not well done, nor were they well received, at least that's the majority of opinion. Having finally seen Showdown myself, I have to agree. Very disappointing.
2) Forgeworld is currently working on its set of WW2 rules. The "unofficial" scuttlebutt is that these are getting the lion's share of attention. That said, I've heard nothing else about them.
Wish I could tell you more. I was supposed to be writing a GW Historical supplement myself and was suddenly out of the loop. Allen Curtis, who posts here occasionally, actually -has- managed to get some of his work published by GW Historical, and to hear him say it, he has no more information than anyone else. (Don't mean to put words in your mouth, Allen, so if I've misrepresented, feel free to take me to task.)
I think the general consensus is this: Over the Top is still theoretically planned, but as Great War has been out for some time without so much as an unofficial army list expansion (which would be a great help), it's not exactly a high priority.
Sorry. Wish I could give you more positive news.
-Doc