ESRI is to soon release ArcGIS for Android which is an application which allows users to use ArcGIS content directly from their Android device. Users of ArcGIS for Android (which will be available from the Android Market) will be able to display and browse maps, query remote ArcGIS map layers and data, use data from ArcGIS online and collect GIS data.
There is also an API as well as an Eclipse plug-in that developers can use to exploit some of the capabilities such as geocoding and geoprocessing of ESRI’s ArcGIS Server from an Android mobile phone or tablet device. There is currently a beta program available for sign up Java developers who wish to beta test the new Eclipse plugin and API from ESRI.
Update: ESRI has delayed their release of ArcGIS for Android from Q2 to Q3 based on “feedback from internal and external beta sites”
Update 2 : ESRI has released version 1.0 of ArcGIS for Android in the Android Market.