Dude, your code is better now but still not fully bulkified... For example, if the trigger fires on more than one task record(more tasks, like in the case of a dataloader operation) you are still doing the select for Cards and the update cardslist for each task. You need to move that for outside of the if:

trigger CCCards on Task (after update) {

	//Preparting CCCards to Update
	public list<CC_Card__c> CardsToUpdate = new List<CC_Card__c>();
	public set<Id> CCCardIDs = new Set<Id>();


	if(Trigger.isUpdate){

		for(Task t : Trigger.new)
		{

			if(t.IsClosed == True && t.subject.contains('App Preview Sent to Customer'))
			{
				CCCardIDs.add(t.WhatId);
			}  

			for(CC_Card__c c : [Select c.Id, c.stage__c From CC_Card__c c where Id in :CCCardIDs])
			{
				c.stage__c = 'Preview & Iterate';
				c.a_Preview_Date__c = system.today();
				CardsToUpdate.add(c);
			}
			update CardsToUpdate; 
		}
	}
}

Also remove one of the time conditions on the trigger, leave just one, either before or after, no need for both...

And yes, the test class. I forgotten about that, sorry :P

@isTest
private TestCCCards {

	static testMethod void myUnitTest() {
		//create a new Card
		CC_Card__c card = new CC_Card__c(Name='test');//put here the required fields
		insert card;

		//create a new Task
		Task task = new Task(WhatId=card.Id, Name='test task', IsClosed=false, Subject='test subject');
		insert task;

		Test.startTest();
		//we simulate the trigger conditions
		task.IsClosed = true;
		task.subject = 'App Preview Sent to Customer';
		update task;
		Test.stopTest();

		//the trigger has fired and we check that the card fields have been modified
		List<CC_Card__c> cardList = [SELECT Id, stage__c, a_Preview_Date__c FROM CC_Card__c WHERE Id=:card.Id];
		System.assertEquals('Preview & Iterate', cardList[0].stage__c);
		System.assertEquals(Date.today(), cardList[0].a_Preview_Date__c);

	}
}

 You run it (from eclipse) right clicking on the file in the package explorer > Force.com > Run Tests.

Thanks,

Adi