I would just like to echo the sentiments of Nick Laurence and Ben Spackman above.
I use Accordance on my Mac and BW (8 for the moment but plan to upgrade to 9 very soon--almost have enough money saved) with Fusion and it is plenty fast, and in fact I prefer the layout for search results (Verse List) in BW over Accordance. Like Nick, I have only purchased Accordance and certain modules to complement BW and would like to see a Mac version with a greater range of modules. I also would be willing to pay a little more for a Mac version of BW, because honestly, I only emulate Windows in Fusion for BW.
That aside, I also watch tech trends carefully and Mike is wise to be cautious. As much as I prefer OS X and various Mac-only programs over Windows for general production, there is a trend toward tablets (and unfortunately for programmers), specifically toward the ARM tablets. We will see if MS's Surface changes any of that; however, it is worth noting that Apple has notified resellers to increase space for displaying the iPad because it currently comprises "55% of dollar share and 61% of unit share among sales at [Apple] resellers but only [represents] 15% of display space." If the iPad mini which is rumored for a September introduction hits the right price point, MS may struggle to gain any significant share of the market.
Also, Mike, I think I read somewhere else that you had new features planned for BW on Win 8--why? Is it possible only because of the new OS architecture? You apparently really appreciate the operating system and I am going to play with the RTM, but I find Win 8 jarring when it switches from the (formerly known-as) Metro UI to the Desktop space. Even with the replacement of the Start button, not ALL Win 7 Start Button functionality is being restored. Perhaps I will look into Crossover....