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I wouldn't see why you couldn't have a blank password. It's a bad idea, of course, and I'm not silly enough to put it to the test. :)
 
When my friend made me a guest account on her iMac G4 (I was dog sitting, and I was considering the switch!) it even needed a password for it.

GRANTED, this was 10.2 (how they haven't upgraded confuses me!)

I'm pretty sure that you NEED a password-- it's the thing that makes os x so super-dee-dooper. (Can't install Apps without password, blablabla)
 
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