I know that ESRI has ArcGIS APIs for Android what other options are there for me as developer, if I'm about to develop GIS applications for an Android phone?
Debateable whether 'ArcGIS for Android' is a fully functional GIS (Geoprocessing, Database etc)
But gvSIG Mini for Android is comparable and open-source equivalent to ArcGIS for Android.
Perhaps this open source tool is also of interest:
"Geopaparazzi is a tool developed to do very fast qualitative engineering/geologic surveys. Even if the main aim is in the field of surveying, it contains tools that can be of great use also to OpenStreetMappers as well as tourists that want to keep a geo-diary.
Geopaparazzi is now available on the Android Market. Search for geopaparazzi on your phone. ... " http://code.google.com/p/geopaparazzi/
Nutiteq Android SDK has now version 2.0 with 3D features and much improved performance, probably worth to look into.
I know very little of Android as such but here is what I do know. Android is Flash enabled and there for if you are willing to play around with it any Flex development environment would allow you to build custom apps for an Android phone.
Apparently Silverlight is in dev for android but I wouldn't hold my breathe for it now. Although this would open up WPF as an application interface.
Have Fun, CDB
What about something like ExtMap Touch? As of today it doesn't support Android, but according to the website - "...it will be working on every mobile device (like Android, BlackBerry, WebOS and Bada) in the future which have webkit-based browsers.". Quite frankly it looks good and if you need ArcGIS Server support it's one of very few choices beyond ESRI's ArcGIS on iOS.
If you're using a Chrome or Safari as a desktop browser you can demo the application. See the link on ExtMap site.