These numbers are a bit different if you try to look at app usage. The problem with blackberry is that not all of this 40% do have the blackberry app store installed. Not all of them can run the same applications and many of the users have not installed a single application since they got their blackberry (apart from apps that come with the phone installed by the vendor or a company giving the blackberry to their employees).
I used to look at the admob statistics. These will show you how much ads are shown on which devices. A figure that could show how much free apps are used on a special OS. The numbers are fairly low. The last report sadly from May 2010 shows only 5% ad market share for RIM.
My personal opinion is if you hope to join a big market of users that will buy your app don't start working in the BB market. If you need the app because of marketing opportunities just to say: we cover iPhone, Android and Blackberry consider building something like a wap web backend or a special website that is compatible with most of the blackberry browsers etc.
It depends what the new OS that will be used on BB Devices is able to do and how loyal the BB users are but at the moment developing end user apps for sale through the app store for me looks like jumping on a sinking ship.