One of the best source for Android on ARM is directly from Google's own Android website. Here you will find links to various resources, references, development kits and the raw ARM sources for the Android project. Androidtools.org is a site for information on both official and alternative tools for developing applications on Android as well as other resources.
Android development on ARM falls into two categories; application development and OS porting to various ARM-based SoCs and boards.
There are two development kits for Android applications. The first is the SDK, or Software Development Kit, which is a Java development environment that produces applications intended to run in the Android Dalvik Java VM. The Dalvik runtime environment, heavily optimized for the ARM architecture, including an ARM targeted Just-in-time (JIT) compiler, utilizes the latest ARMv7 architectural features.
The second development kit is the NDK, or Native Development Kit. The NDK augments the SDK for the creation of native ARM code used in timing sensitive or performance critical applications. If you are developing Native code and in need of a free, powerful and user friendly debugger, take a look at ARM's DS-5 Community Edition. DS-5 CE is an Eclipse pluggin that work along side your existing Android SDK and NDK tools to add not only improved debug support but also Streamline, a powerful performance analysis tool. For more information visit our Advanced App Development Resources.
If you're looking for information on low cost software development hardware, feature rich Android devices for hardware development, reference designs or Android porting guides and partners, please visit our Android Handsets and Device Developer area.
Creating a new "port" of Android to a custom ARM based SoC or development board requires the Android platform sources. The Android sources are made up of a collection of git repositories consisting of unique Android open source projects as well as readily available external projects. Google has created a tool called "repo" to manage the numerous git repositories that make up the full platform. Getting started with repo and the Android sources is simple and well documented on the Android sources portal.