What percentage of Bitcoin transactions never complete?
I have watched for reports of a transaction that appeared live on the net that never got into a block. It has never happened, and I don't think it's likely to happen. The only practical way for it to happen is for the network to be split due to a connectivity problem or for a miner in cahoots with the person attempting the double-spend attack to mine a conflicting transaction (not accepted by the network) into a block before the transaction itself gets into a block.

I agree that accepting 0/unconfirmed transactions is sufficient for point of sale applications. Someone is very unlikely to attempt a double-spend in person for a few thousand dollars or less and is even more unlikely to succeed. (Update: Due to the risk of the Finney attack, I withdraw this argument.)

I think your guess is only reasonable within limits though. For big transactions, companies will sue people who charge them back. They can do the same thing with Bitcoin transactions if they ask for ID first. For face-to-face transactions, I don't think confirmations matter for transactions that aren't massive.

It's not a big deal for online transactions for a different reason. If we're talking a small payment for access to a web site, is anyone really going to bother with an attack? And does anyone care? Here, the percentage risk argument works. And for big online stuff, delivery's usually not immediate anyway (think PayPal, eBay, Amazon).