I very much doubt they would still be allowing 0-confirmation bets if they were seeing such an attack happening.
There is a person reporting success in two of three attempts to double spend against the site.
The tactic is to create the initial spend using a spam-like transaction (e.g., over 1K of data, from lots of small transactoins) and pay no fee.
That transaction will not confirm anytime soon -- perhaps for many hours.
If the bet ends up being a losing bet, then from a another client, create the double spend transaction and include a nice fee. The client will transmit and rebroadcast the double spend transaction. Most peers won't accept the transaction as it is a double spend, others do and also will try to relay it. Eventually a miner (apparently using a custom client that will prefer the replacement transaction with the higher fee) will mine a block that includes the second (double-spend) transaction.
Here's the thread on that:
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https://bitcointalksearch.org/topic/m.1401014