This will only apply to ArcGIS for Desktop 10.1 and later but at those versions the [Assign Domain To Field (Data Management)][1] tool is available and ...
Sets the domain for a particular field and, optionally, for a subtype. If no subtype is specified, the domain is only assigned to the specified field.
To obtain an iterable list of subtypes for a field arcpy.da.ListSubtypes can be used. [1]: http://resources.arcgis.com/en/help/main/10.1/index.html#//00170000001w000000