Tracking transactions as a Receiver I have a question concerning the transaction of bitcoins. If, for example, I send some BTC to a specific address, does the receiver have any chance in verifying that I have already sent the BTC, before they are being registered in his wallet? I know there is http://blockexplorer.com/, where I can look the transactions up. But is there any functionality for it in the Bitcoin-Client, which I could use in RPC-Calls?

Use case for this: Assume there is an application that uses Bitcoin via RPC-Calls to the Client. I want to buy something via the application and the seller should be able to see that I paid the correct amount of BTC without having to wait for the transaction to be finally redeemed in his client.

Thanks in advance!
The client will often know about received transactions seconds after they have been sent. Transactions that have not gotten into a block yet are said to have 0 confirmations. You can see them by adjusting the parameters of listreceivedbyaddress, etc. so that they show 0-confirmation transactions.