Open source Operating Systems (OSes) and the web have effectively rendered operating systems irrelevant. Microsoft was so busy milking its cash cow Windows OS that it failed to see this and is stuck in a legacy net. Sure, it profits, but only because businesses (the bulk of Microsoft's profit base) move slower than technology. Consumers have already forgotten about OSes. All they need are apps.
Apps are the excellent middle space between OS and web. OSes are device dependent: you can't run a Windows app on a Mac without some emulation; a blackberry app will not run on an iPhone. The web is network dependent: you can't run anything unless you are connected. Apps are a hybrid: beneath that icon is some local functionality and some web-based functionality. Some apps are little more than a pretty bookmark to a website. Some apps are mostly offline (like games), with a small online component (eg. posting high scores to Twitter).
If you're going to build an app today where do you start? Apple has the bigger, more profitable app market, but Android has more devices out there. And what about the smaller players: Windows Mobile and Blackberry? I believe the answer is simple: the web.
Since most apps have an online component anyway (particularly if you're looking to exploit cutting-edge in-app purchasing), you might as well start there. The technology platform is ubiquitous - every phone, tablet and laptop has a browser - and most web analytics track what browsers are accessing your site, so once you generate a crowd, you can then decide, based on actual evidence, which app channel you're going to develop for first.