While on paternity leave, I wrote this first Android application as a kind of remote baby monitor - even though I would never dare to suggesting using it as such in public... The basic functionality is that when running the application monitors the sound volume through the microphone and if it exceeds a certain threshold automatically dials a number which previously has been configured.
Based on existing examples which come with the SDK distribution it is pretty easy to put together the basic application skeleton - including the XML description of the UI layout. If you don't know what you are looking for the documentation on the Android SDK site is a bit sparse but is great to look up details.
The application consists of an Activity, the Android abstraction for a single self-contained screen with some associated behavior and user interaction. This screen has a layout and a main menu defined in XML and provides framework methods to handle I/O and life-cycle events. For simple apps we can also use periodic event handlers to trigger actions directly from the UI thread.
It seems the only way to currently measure the sound input at the microphone is to run the MediaRecorder and use its getMaxAmplitude method to periodically get the maximum sound level since the last call. Setting up the recorder to write to "/dev/null" - the Unix special bit blackhole works just fine, which avoids filling up the flash card and having to deal with cleanup. For added benefit, there is a special graphical widget which displays a bar power meter with the current sound level from 1-10 in green and red depending on the threshold.
When it comes to calling the phone number, Android supports a intents - a framework of publish-subscribe signals which an application can send to get something done without knowing who is registered to handle them. In this case, the standard system phone application happens to be registered for the ACTION_CALL intent which is used to call a number.
The configuration is nearly automatic. A hierarchy of preferences can be described in XML and then loaded to generated a standard preferences screen which happens many common cases: on/off checkbox options, 1 out of N selection lists, text entry fields, etc. When the preference screen returns it has automatically store the results in the applications default preferences DB instance.
Thus writing a simple Android application with a main menu, some settings and a bit of behavior is really that simple.
The application code is open-source and can be found at http://code.google.com/p/android-labs/source/browse/trunk/NoiseAlert/. Application package can be downloaded from the same site or is available through the Android Market.