Not really, the address is either used (for payments) or not. It wouldnt be in a state where both is equally likely.
if 2 people have an cat (same address) in a box (wallet address pool of 20-100 addresses yet to be used) but cannot yet see if its an active address ready to jump out and walk around (collide and be spent by other person) or not.. ... have they yet collided..
my point is that you can never know until you do something.
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Hmm, makes sense seen like that.
Gmaxwell and others have been advising against brainwallets for a reason... they are not and will never be safe.
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That was not the reason, the reason was and is that humans are bad at producing sufficient entropy for the brainwallet to be random (see the post by franky). This is true for the security of all wallets, if the entropy source sucks a collision is (significantly) more likely.