If you take 1 BTC and create two transactions, both spending it to different addresses, then send out both transactions simultaneously to half your peers each, you could likely get two (or more, expand multi-spends to as many peers as you have) groups of mining peers on the network to promise to include each individual transaction in direct competition with each other as the announcements would arrive across the network at varying times.  As the spender, you are attempting multiple transactions at once and hoping that the recipients deem the number of promises they receive for their individual transaction to meet their threshold for trustworthiness.  As the recipient, this threshold would likely need to be a large majority of the network mining peers so as to ensure that even at the minimum number of possible double-spend transactions, two, you are reasonably sure that your transaction will be the one to be included in the block-chain first, thus preventing any duplicates from double-spending what you've received.

However I guess if the recipient client detects duplicate transactions on the network after a short given period of time (far less than waiting for a block to be found), destined for multiple different recipients, it could just default back to waiting for block confirmations.