How about adding a feature to the Bitcoin Client that would upon receiving or confirming a payment, send POST data to a URL that includes the amount received, address sent from, address sent to, and time received.
Also, possibly also a feature doing the same for outgoing payments.
Possibly an easier solution than API through a 3rd party.
Have a block at blockexplorer.com. This site keeps track on all blocks and all transactions.
Best regards,
mnemonix