$skuArray = array();
$categoryCollection = Mage::getModel('catalog/category')->getCollection();
foreach ($categoryCollection as $category) {
$skuArray[$category->getId()] = $category->getProductCollection()->getColumnValues('sku');
}
$categoryCollection = Mage::getModel('catalog/category')->getCollection();
foreach ($categoryCollection as $category) {
$skus = $category->getProductCollection()->getColumnValues('sku');
$category->setData('skus', $skus);
}
foreach($categoryCollection as $category) {
$categorySkus = $category->getData('skus');
}
$categories = Mage::getResourceModel('catalog/category_collection');
foreach ($categories as $category) {
$products = $category->getProductCollection();
foreach ($products as $product) {
$sku[$product->getId()] = $product->getSku();
}
}