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RAM spec does seem conspicuously absent. I assume that info is available to developers. I haven't gone to the development site to take a look yet, but once I do I won't be able to talk about it. :(
 
I would guess 512 MB of RAM. 256MB is what the 3GS has right now. However, a more enhanced version of the iPhone OS, would require more breathing room. It could have between 512 MB to 640 MB.
 
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