Can one send bitcoind transactions that will only be valid after a certain amount of time? The standard client only allows one to spend coins from a transaction after getting 6 confirmations (120 in case of mining rewards). Can one send bitcoind a JSON-RPC command telling it to send those unconfirmed coins before they gather enough confirmations, so the transactions would be executed once those confirmations appear (essentially telling it "once you get enough confirmations, send my coins to X")?
The bitcoin.org client will allow you to spend coins after just one confirmation has been received.

The Bitcoin protocol has no restriction that would prohibit you from spending immediately (o/unconfirmed) so it simply would be up to the client supporting it.   

The My Wallet from http://www.blockchain.info/wallet  supports spending immediately after receipt.