If you're a BlackBerry application developer who is building apps for the BlackBerry OS then it is pretty crucial to know where the majority of your potential users are. That said; Research In Motion has just put some great charts outlining data that puts a few things into perspective.
You'll need to know which OS' the majority of your users are going to be working off of. Much of that decision will be based off of the fact whether or not your app will be that of a free one or a paid one. If you are working on paid app, then of course you'll not want to be working on an OS that is no longer supported and most people downloading apps are only making use of free ones. You would want to target the audience with current supported OS' and the one where paid apps leading over top of free apps to maximize sales and leverage use of the newer system integrations and tools.
The landscape for BlackBerry Developers as we know it is changing -- with the launch of the PlayBook we're getting amplified look at the way BlackBerry Development is headed. But until then, knowing which BlackBerry Smartphone and audience to cater to is still a big part of day-to-day operation in developing BlackBerry apps. Head on over to the BlackBerry Developers page to get a further look at the numbers.