At the moment, there's a huge opportunity for small, independent developers to make their millions by making applications for smartphones. So if you can handle a bit of code, this is how you get started for each of the main smartphone platforms.
If I were a developer, I'd probably start with Windows - it's already got a fairly large userbase, yet their Marketplace is fairly barren, at least compared to the millions of apps on the iPhone. Coding for Windows Phones is similar to coding for Windows proper, so most people with coding experience should manage fairly easily. That said, there's a certain level of doubt over the transition from WM6.5 to WM7, so what you develop not may not work on future versions of the OS.
To get started, point your browser at marketplace.windowsphone.com. You'll need to register using a Live ID, as well as providing business details. You'll also have to pay a $99 yearly subscription fee, but from there, you'll start receiving 70% of the revenue from sales of your app.
The iPhone changed the way we thought about mobile phone software, and as such its the most successful App Store by far. The problem with such success though is that there's a very good chance your app will get lost in the millions of other apps on the store, so make sure you make your effort stand out.
You'll need a Mac running Leopard to develop for the iPhone, so once you've got that, head to developer.apple.com/iphone. You'll need to register or log in with an Apple ID, then agree to the EULA. It costs $US99 to join the developer program, and from then you walk away with 70% of the revenue from your app sales.
As an open source platform, Android allows some of the most versatility for developers, but there are still some restrictions. The SDK is available for Windows, Mac and Linux, as an example. Head to developer.android.com to get started.
You'll still need to pay a registration fee - which is $US25 - and then you'll receive 70% of the revenue from you app sales. Remember though that Optus still doesn't permit the purchase of paid applications through the Android Market, so that's potentially a lot of lost revenue right there.
The once powerful Symbian mobile OS has fallen well and truly behind the other platforms for smartphones, but Nokia is pushing its return with the Ovi Store. If you still love the Finnish handset maker and want to market to their massive international audience, you can develop for Symbian.
Publishing to the Ovi store costs €50 (about $80) and you receive 70% of the revenue. To start your Symbian development career, visit forum.nokia.com/ovi.