Thanks for the reply.

Unfortunately I get a mix of messages like "Warning: array_key_exists() expects parameter 2 to be array, null given in /var/www/post/include/template.inc on line 17 Warning: Invalid argument supplied for foreach() in /var/www/post/include/template.inc on line 25" and "You are not allowed to use this service!" (from http://ceptacle.com/cash). Still, it seems like the overall idea is somewhat similar to what I had in though, although I'm not sure how the user would be able to administrate his wallet. Wouldn't it be possible for malicious merchants to steal coins (more than what you are willing to send to them) by tweaking the code on their site?

I was thinking more in the direction of a browser extension that connects to some online wallet, like blockchain, through their API (eg. https://blockchain.info/api/blockchain_wallet_api). When properly configured, it would pop-up when users click payment links (bitcoin:[address]), ask for confirmation (and amount, if not encoded in the link). All payments would be denominated in bitcoins to avoid exchange rate and bank issues altogether. Since the user retains full control of his wallet (assuming you trust the online wallet provider), only the (small) amount he agree to send to the merchant could be stolen.