Apple's (AAPL) iPhone business is great for Apple: It's providing the company's strongest source of growth, it's a technology leader, and it's very profitable. But it's still very small, all things considered.

Mobile research firm ABI Research estimates 258 million mobile phones were shipped worldwide in Q1. With 3.8 million March quarter shipments, Apple's iPhone represented just 1.5% of the market, if ABI's estimate is accurate.

That means that there's lots of room for Apple to grow. But it also means that the vast majority of the world's population are choosing non-Apple phones. Not all of them have the opportunity to buy iPhones, but many do.

We believe that the mobile market is going to become more like the desktop market: As software becomes more important, people will be more loyal to mobile platforms. BlackBerry users will be more likely to stay with BlackBerries, iPhone users will be more likely to stay with iPhones, etc. So at some point, Apple will want to build its market share.

Yesterday, we outlined a few things Apple should to do to keep its iPhone business growing. And you left some great comments.