which are perfect valid if using the shell:
> db.po.insert(

{"":1}

);
> db.po.insert(

{"":2}

);
> db.po.find(

{"":1}

);

{ "_id" : ObjectId("4e3940ff933089c859e4d4f9"), "" : 1 }

error source:
private void _checkKeys( DBObject o ) {
for ( String s : o.keySet() ){
if ( s.contains( "." ) )
throw new IllegalArgumentException( "fields stored in
the db can't have . in them" );
if ( s.charAt( 0 ) == '$' )
throw new IllegalArgumentException( "fields stored in
the db can't start with '$'" );
Object inner;
if ( (inner = o.get( s )) instanceof DBObject )

{ _checkKeys( (DBObject)inner ); }

}
}

if ( s.charAt( 0 ) == '$' )
should be:
if (s.startWith('$' ))