You bring up a great issue! However, I wonder if there are other ways to extend your reach to all of those other platforms without having the expense of deeply developing on all the minor different platforms. Cozi does have a passable mobile website, the question I would have is why not build out the mobile website to a higher degree (think what Facebook did with touch.facebook.com) that would generally render well on all of the different smartphone platform out there? However I think part of your motivations is the ability to be "featured" in the platform's application market and thus having a quick and available shortcut to the service. Obviously prominent placement in the Apple App Store did wonders for Cozi :-) ... This could be addressed by creating a "wrapper" application that adds some additional on device functionality onto of the mobile web site which would be embedded into the app. In theory this could mean a small amount of development work on these minor platforms, but wouldn't necessarily require making a "HUGE" investment especially since you didn't even consider Meego in addition to Windows Phone 7, Palm, and BB. But of course life is simpler with only 2 contenders since it helps to reduce the number of tools needed. But HTML apps really can be cross platform to a decent extent and the heart (brains?) of the app can be something that could thus be shipped out to as many platforms as needed -- assuming that the platforms continue to (and improve) the ability to render it.