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Wow I can't wait for 4.2 now , it seem sooooo much faster than the ios4.1 that I'm on which I hate.
 
Yes, you need to be on a registered dev account to install these IPSW's. iTunes will error out if you're not. They do check UDID.

Not if you update from the file. Only if you restore does it need to be registered because it looks to activate the phone...
 
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