I don't think anybody knows. But you can find out:
Send a token amount of bitcoin to two more brainwallets, made from each of the sentences.
When somebody else either spends those bitcoins or sends more bitcoins to them, you know that your main brainwallet isn't safe any more. (are the services that will send you an email when there is activity on a bitcoin address still around?)
So, there is a flaw with that strategy, Gavin: A shrewd attacker who is systematically scouring passphrase space for coin balances might intentionally pass up most of the smaller, more easily-found jackpots so as not to alert the community that he has this search capability; he may be waiting to pounce until he finds a single, sufficiently-large stash. Kind of like how the British code-breakers in WW2 intentionally did not act to prevent many U-boat attacks because they didn't want to tip off Germany that Britain had broken the Enigma code. A classic application of information theory to warfare...
Furthermore, if your short "test" passphrase is a substring of your real passphrase, then by using it as a probe, you've now actually made it easier for attackers to figure out your longer, real passphrase... Since once they find the shorter passphrase with coins in it, now to find the long one, they only have to search the subspace of longer strings that includes the shorter one as a substring.
And including ID information (as you suggested earlier) could be counterproductive if the attacker has the capability to trace the transaction graph back to Mt. Gox (say) and compel them to release the customer's dox... Then they can easily include the ID info when searching for the longer string, AND further they will have evidence that the big stash still belongs to you when they do find it.