With the Winter '12 release, Salesforce.com support APEX SOQL only (not DML) across
the Assignment and Permission set objects which includes User Permisions :(
To know other standard objects which doesn't support DML operations, kindly use the link below
http://www.infallibletechie.com/2013/09/sobjects-that-dont-support-dml.html
To know other standard objects which doesn't support DML operations, kindly use the link below
http://www.infallibletechie.com/2013/09/sobjects-that-dont-support-dml.html
