UPDATE: This works on 1.5 as well

I was working on a project that imports mass products into magento based on a table in a MySQL Database. If you are looking to create a script to do this, you can easily accomplish that with a script like the following (of course with some stuff to like.. load your data, and maybe and a foreach loop :p ):

// First we load Magento. This applies to loading magento if you are  not going to be making some sort of extension/module to accomplish this for you.

require_once('/path/to/magento/app/Mage.php');
umask(0);

// Set an Admin Session
Mage::app()->setCurrentStore(Mage_Core_Model_App::ADMIN_STORE_ID);
Mage::getSingleton('core/session', array('name'=>'adminhtml'));
$userModel = Mage::getModel('admin/user');
$userModel->setUserId(1);
$session = Mage::getSingleton('admin/session');
$session->setUser($userModel);
$session->setAcl(Mage::getResourceModel('admin/acl')->loadAcl());

// Then we see if the product exists already, by SKU since that is unique to each product
$product = Mage::getModel('catalog/product')
   ->loadByAttribute('sku',$_product['sku']); 

if(!$product){
  	// product does not exist so we will be creating a new one.
    // i noticed some fields will give a database error if they are set when existing. These are the fields I found to work successfully

  	$product = new Mage_Catalog_Model_Product();

  	$product->setTypeId('simple');
	$product->setWeight(1.0000);
	$product->setVisibility(Mage_Catalog_Model_Product_Visibility::VISIBILITY_BOTH);
	$product->setStatus(1);
	$product->setSku('UNIQUESKUHERE');
	$product->setTaxClassId(0);
	$product->setWebsiteIDs(array(0)); // your website ids
	$product->setStoreIDs(array(0));  // your store ids
	$product->setStockData(array(
		'is_in_stock' => 1,
		'qty' => 99999,
		'manage_stock' => 0,
	));
}

// set the rest of the product information here that can be set on either new/update
$product->setAttributeSetId(9); // the product attribute set to use
$product->setName('Product Title');
$product->setCategoryIds(array(0,1,2,3)); // array of categories it will relate to
$product->setDescription('Description');
$product->setShortDescription('Short Description');
$product->setPrice(9.99);

// set the product images as such
// $image is a full path to the image. I found it to only work when I put all the images I wanted to import into the {magento_path}/media/catalog/products - I just created my own folder called import and it read from those images on import.
$image = '/path/to/magento/media/catalog/products/import/image.jpg';

$product->setMediaGallery (array('images'=>array (), 'values'=>array ()));
$product->addImageToMediaGallery ($image, array ('image'), false, false);
$product->addImageToMediaGallery ($image, array ('small_image'), false, false);
$product->addImageToMediaGallery ($image, array ('thumbnail'), false, false);

// setting custom attributes. for example for a custom attribute called special_attribute
// special_attribute will be used on all examples below for the various attribute types
$product->setSpecialAttribute('value here');

// setting a Yes/No Attribute
$product->setSpecialField(1);

// setting a Selection Attribute
$product->setSpecialAttribute($idOfAttributeOption); //specify the ID of the attribute option, eg you creteated an option called Blue in special_attribute it was assigned an ID of some number. Use that number.

// setting a Mutli-Selection Attribute
$data['special_attribute'] = '101 , 102 , 103'; // coma separated string of option IDs. As ID , ID (mind the spaces before and after coma, it worked for me like that)
$product->setData($data);

try{
	$product->save();

} catch(Exception $e){
	echo $e->getMessage();
	//handle your error
}

I found how to do this on various pages such as some answers in http://www.stackoverflow.com and as well as some topics on the magento forum. However there are bits and pieces I got from some other websites which I did not bookmark and am sorry if I leave you out of the following links as I just put this up cause I though it might be useful for someone looking to add products with lots of attributes and pictures and I just kinda copied stuff out some of my code for example purposes.

http://rocinantesoftware.blogspot.com/2009/06/programmatically-importing-product-with.html

http://activecodeline.net/programatically-manually-creating-simple-magento-product