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I don't think a 64-bit photoshop is really necessary right now. I mean, hardly anyone is using the 64-bit version of windows XP/Vista because noone wants to have their apps support it, and apple hasn't even released Leopard which is supposed to finally fully support 64-bit, so what is Adobe supposed to do? I think apple might make it easier for 64-bit app makers in the future by adding another 64-bit binary to the Universal binary, so that way if the application recognizes a 64-bit machine it will run there. Until that happens though....
 
2D applications gain little by becoming 64-bit applications, though the size of the colour space is increasing, as many scanners support 48-bits now. The size of digital photos may make the conversion necessary some day in the future but not at this moment.
 
from what i have gathered so far, the performance of a 64-bit CS3 is not equal to the hype that surrounds it. and from a business perspective, there not enough current 64-bit users to justify the cost of development this cycle.
(see below link for details)

thanks for the follow-up links and here is another from an adobe engineer.

http://blogs.adobe.com/scottbyer/2006/12/64_bitswhen.html
 
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