At this stage there is absolutely no plan for a windows 7 version, it's just not going to happen.
As for Windows 8, we're taking our usual wait and see approach, but they would have to take off in a BIG way before we make an app for it.
Just to throw some numbers around to help show why:
Assume there's 100,000,000 (one hundred million) iPhones in use now (this is probably significantly understated)
Assume there's 20,000 copies of YNAB running on iPhones (I don't know the numbers at all here so this is purely a guess)
That means that one in every 5,000 people with an iPhone have bought YNAB, at a cost of $5 = $100,000 (like I said, this number is based on some terrible assumptions, but you can see that it isn't that much money - especially after Apple takes 30%!).
As of December 2011, there were 5 Million Windows 7 phones sold. Assuming they've doubled that now, that would mean we'd be working from a base of 10,000,000.
If we assume the same level of YNAB apps sold (1 in 5000), then we could possibly have 2,000 copies of YNAB sold (and this is a HIGH estimate). At $5 a piece, we're talking $10,000 total revenue. At best! It would probably be significantly less.
I'd imagine we're actually losing money on the iPhone and Android apps at the moment based purely around those guesses above (and I haven't seen the numbers for those apps, so I could be wrong), but they are so useful, and make the desktop app work so much better that we want to keep developing them. So looking at these (very simple, and wrong) numbers, you can see why we are very much waiting to see what happens with the new Windows 8 Phones. $10,000 is not much money at all when you are talking about hiring a developer full time to get the app built and integrated with the desktop/cloud sync, along with support and bug fixes, and ongoing improvements and development. It might still happen, but I wouldn't hold your breath!