I am trying to set up a system that will post feature objects periodically to a PostGIS db and display them via GeoServer and OpenLayers on a web site. Is it possible to add the new feature objects to the db via GeoServer using the REST API or geoserver-manager (my system is Java based) or will I have to add the features directly into the PostGIS db?
If you are adding the data from a client then you should probably use WFS-T - GeoServer will then "know" to update any cached tiles that include the layer. However if you are collecting the data from (or via) a process on the server then doing a direct write to the database will be more efficient (saves the change to GML and back). You might find GeoTools helpful for talking to PostGIS databases.