there are lots of different options when getting in to mobile development - iOS, Android, Blackberry, Windows, and there are always lots of arguments for the pros and cons for each platform. For me, here are a few things that make Android a good platform to get into:
1. Barrier for entry is low: you can develop on pretty much any desktop/laptop (obviously as long as its powerful enough) and has very good IDE support (Eclipse is open source so free and has good Android support)
2. As mentioned in Ramon's links - Android has a quickly growing market share of the mobile market
- Windows Mobile OS is faltering and has the disadvantage of coming late to the game with a serious mobile platform (Windows 7 mobile is the first serious contender, earlier Windows mobile platforms were not up to much)
- iOS & Blackberry are limited to proprietary devices so are directly limited to the sales of those handsets, whereas Android is being used by more and more mobile companies (LG, Samsung, HTC, etc all have Android devices) and are available on a range of handsets & prices. (Although, you should not under estimate the market share that is held by apple/BB)
- Symbian, Nokia has all but dropped this recently with a move to using Windows 7 on their future devices.
At the end of the day, I think there are a couple of good platforms, that aren't going to disappear, so you can do well with many of them - if you have an Mac and an iPhone, then iPhone development is a good option (you can *only* develop iPhone apps using a mac and iPhone)- but otherwise, i personally think Android is a good horse to back and requires limited up front investment to get started.