I still think Christmas would make a whole lot more sense for such a deal. After all you've got the end of XP support AND the start of the new FY in April, so ideally you want a deal out before then - and Christmas (or perhaps New Year) is the obvious point at which to do that.
Why did I say XP? Simply because it's going to be a LOT easier "sell" to get someone from XP to 8 than from 7 to 8 - based on the lengthy discussions here at least. The ending of official support is also a powerful incentive for those who can, but still haven't, moved from XP.
Christmas makes more sense to me, because then you're maximising your advertising spend - it stops being "buy a new PC with new Windows 8" and becomes "get 8 - either upgrade or buy a new PC".