Good question. Instead of having a unique download for each developer they could have each developer enter a unique key when they install/activate the software. When that key is entered they could then either sign/watermark the necessary files and/or use that key to sign/watermark the images as they are drawn or capped.
That way they wouldn't have any unique downloads and it would require the person to circumvent a copy protection mechanism (ahem, DMCA) in order to get around it... if they even realized it was there. They might be harder to track down, but if they were Apple would be in a much better position to go after them on DMCA grounds as well as NDA violation.
Yes, but that would defeat the whole purpose of their developer seeds. You are implying that they would have some authentication built into the SnowLeopard dev seeds, and then remove it when they go gold. Well even the removing of this authentication piece could cause unforeseen problems after its removal that Apple would want to test (the whole point of seeding!)
The only changes that Apple wants to make between the last seed and master that is pressed to the DVD is to add any last 'graphic' look in the form of image or movie files. (The new side dock, and the intro space 'Welcome' movie in the GM of Leopard)
Ultimately, they would have to be looking at implementing a system-wide "validation" piece that would be present to the consumer as well, which I don't see them doing.