The obvious question is when this property is set. After product is loaded it is already set on model but it is not an attribute and is not a column in product flat table.
As usual, all the bizarre stuff in Magento is done by observers. Mage_Cataloginventory is observing catalog_product_load_after event and there it comes down to Mage_CatalogInventory_Model_Resource_Stock_Status::getProductStatus and the following query:
SELECT `cataloginventory_stock_status`.`product_id`, `cataloginventory_stock_status`.`stock_status` FROM `cataloginventory_stock_status` WHERE (product_id IN('241319')) AND (stock_id=1) AND (website_id=3);
It is clearly visible that the decision if product is salable or not is made during reindexing. And disregard stock_id which is sort of unfinished functionality which will also pop out later.
So we are ending up in a place no sane Magento developer will willingly go .. the indexer. Catalog inventory indexer in our case. After quick travel through the maze of Mage_CatalogInventory_Model_Indexer_Stock::_processEvent, Mage_Index_Model_Indexer_Abstract::reindexAll and Mage_CatalogInventory_Model_Resource_Indexer_Stock::reindexAll we discover that each product type has it’s own stock indexer which resides in app/code/core/Mage/CatalogInventory/Model/Resource/Indexer/Stock.
Each type has a _getStockStatusSelect method where an SQL query finally decides if the product is in salable or not. Even though the query may seem massive the logic behind is not complicated.
The big part of code here is again this rudimentary stuff. Seems like core developers made a fine attempt to allow having different stock levels for different websites but for some reason this functionality was never finished.
So for instance the check of simple products stock availability only contains of verifying that product is enabled and quantity is positive spiced with a stock management flags. Queries for configurable and grouped products varies a bit due to product type specifics.
And this is the story behind the modest isSalable() method.