Not exactly. It's reliant on whether or not your transaction remains part of the permanent blockchain. There are a few ways it may not, even when appearing valid at first. As a rule of thumb the deeper a transaction is buried in the blockchain the more likely it is to be permanently valid. A simple way to think of it is, 0 confs is fine for trivial amounts you don't care about being reversed, say a dollar or two. Higher amounts should have at least 1 conf, which is about 1 million times safer than 0 confs. Each additional block adds several degrees more safety, and after 6 it's safe to consider the transaction permanent (under normal network circumstances).

To understand more you'd need to know a bit about networking and Sybil attacks, as well as how mining works. The problem is you can't call VISA to determine if a txn is valid. A computer network provides all arbitration, and there are ways clever thieves can manipulate that situation to their advantage. It's not generally worth it for a cup of coffee, but for higher amounts it could be. That said, if you have a connection to blockchain.info, which seems well connected, and see a valid txn come through you can be relatively sure it's okay and will confirm, especially for smaller amounts. There are projects (like greenaddress.it) looking to make 0 confs quite safe, by being based on something other than normal blockchain txns.