Question abount getting the sender's address in a transaction

[Cerebrum]

So, I've been looking around at the json-RPC calls in the forum and at the documentation for the server daemon and maybe I'm derping terribly, but I can't figure out how to get the sender's bitocin address when I receive a transaction. I'd like to create an exchange service, but I would need the sender's bitcoin address for each transaction, so could someone explain to me how this is done?

[Pieter Wuille]

See http://bitcointalk.org/index.php?topic=4220.0 and many threads before that one.

[JollyGreen]

Here is my other thread,
http://bitcointalk.org/index.php?topic=4140.0

I've got most of the code to implement this addition completed, but I need to update it to use sipa's latest modifications.  I'm waiting on his code to be approved into the main bitcoin codebase.