I've added an API to Strongcoin aimed at merchants who want an easy way to integrate instant Bitcoin payments into their site.https://strongcoin.com/api
1. I generate you a unique Bitcoin address.
2. User makes payment to that address.
3. A soon as I detect the transaction I notify you of the sale.
Payment notifications are instant so the API is potentially open to a double spend attack. See details here http://bitcoin.stackexchange.com/questions/722/has-there-ever-been-a-successful-double-spend-attack-on-the-bitcoin-network-if
However I believe instant payments give a better experience to end users and that there are techniques we can use on the server side to make the risks negligible.
You can see an example of the API for bitcoin instant micro payments at http://strongdemo.herokuapp.com
, click on a price to see a nice QR code popup. The popup will turn into a download link as soon as you make payment.