You currently are only prompted that a fee will be included if one will be required by the Bitcoin network. If there was no prompt, then your transaction was high priority (made of a sufficient amount of Bitcoins that have been in your wallet for a while), and I would expect that it will be included in one of the next blocks.
There is an open issue to include a prompt, to encourage more voluntary fee and also to confirm that the option hasn't been mistakenly set. I've already made a thread of what that could look like:
https://bitcointalk.org/index.php?topic=143881.msg1525734#msg1525734