This should work, I've included the test class.  I rewrote something I did for updating accounts based on related tasks so you may need to tweak it before it compiles.

trigger UpdateAccountRequester on Task (after insert) { for (Task t:System.Trigger.new) {if (t.subject=='ACCOUNT PACKAGE APPROVAL') { Lead LD = new Lead(Id=t.LeadId, CustomLeadField__c=t.Status);update LD; } }} //////////////////////////////Test Class////////////////////////////////////////////////////// /** * This class contains unit tests for validating the behavior of Apex classes * and triggers. * * Unit tests are class methods that verify whether a particular piece * of code is working properly. Unit test methods take no arguments, * commit no data to the database, and are flagged with the testMethod * keyword in the method definition. * * All test methods in an organization are executed whenever Apex code is deployed * to a production organization to confirm correctness, ensure code * coverage, and prevent regressions. All Apex classes are * required to have at least 75% code coverage in order to be deployed * to a production organization. In addition, all triggers must have some code coverage. * * The @isTest class annotation indicates this class only contains test * methods. Classes defined with the @isTest annotation do not count against * the organization size limit for all Apex scripts. * // TO DO: implement unit test /** *Insert two accounts, one with RP Active and *One With RP Inactive. Insert an Opportunity *and set o.rpaccount field to first Account *and verify system.assert does NOT equal */ /* * See the Apex Language Reference for more information about Testing and Code Coverage. */ @isTest private class AcPkReqeTester { static testMethod void myUnitTest() { /* pssst, lifted this from ... http://community.salesforce.com/sforce/board/message?board.id=apex&message.id=11984 */ //First, prepare opportunity for the test data Lead Lead1 = new LEad(name='test account1'); insert Lead1; Account Lead2 = new Account(name='Not Updated'); insert Lead2; //System.debug('Created Account with Id: ' + acct1.id); //acct.Health_Status__c = 'Customer'; //update acct; Task TaskToCreate = new Task(WhatId=Lead1.Id,Status='YOURVALUEHERE'); insert TaskToCreate; ; // Verify correct action Lead LeadUpd = [select id, Status from Lead where id = :Lead1.Id]; //System.debug('THE VALUE OF HEALTH STATUS IS: ' + 'status'); System.assert(acctUpd.CustomField__C == TaskToCreate.Status; System.assert(Lead2.CustomField__c == null); } }