As far as I can tell, there's no CLI call that enables the client to get the sender's address when the CLI is the receiving account/address. I might be wrong, since the wiki hasn't been updated since 0.3.20.2, but there's no call that I can find that allows you to do it. I wrote a simple workaround, but it would be nice if there was an easy command that I've glazed over somewhere.
The transaction input addresses are available, but they may not correspond to the actual entity who ordered the bitcoins sent to you. For instance, if you send bitcoins to your wallet from an exchange or online wallet service like CampBX or Instawallet, then the sending addresses belong to the web service, not you, and are semantically useless for determining the actual person who sent the bitcoins.