The Sensor Observation Service (SOS) extension for ArcGIS Server (10.1) is implemented as a 'Server Object Extension' (SOE).
It allows querying of observations, metadata about procedures/sensors, as well as descriptions of features (of interest) observed by the sensors.
The SOE's interface provides compliance with the SOS 2.0 standard
from OGC
as well as the new GeoServices REST API developed by ESRI.
More information to this project can be found on its website.
In order to retrieve responses with the HTTP-Header Content-Type set to application/xml, a request must define the URL parameter f=xml (instead of e.g. f=pjson).