Could it conceivably take FOREVER for a zero fee transaction to process?!? I have a couple of small transaction from 8 July that have STILL failed to post to the blockchain. I'm not sure what gives with those ... but suspect they were sent during the SPAM attack and got linked with the spam transmissions?
Btw, I didn't send these transaction, I am the recipient.
zero confirmation just means that no miner has included that transaction in a block yet. miners includes transactions in blocks because the fees goes to them. so when you don't pay the fees, you gotta wait for some rare miners that will eventually include your transaction on their block.
that's just the simple explanation. check
this and
this for the more technical explanations.