It currently isn't possible to boot SL on a P4 or any other older Intel CPU.

As soon as Apple release the XNU sources, then we can start work

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My Specs
System: Custom Built
Mobo: ASRock Z68 Extreme4 Gen3
OS:Mac OS X MountainLion 10.8.2 & Windows 7 Ultimate x64
CPU: Intel Core i7 2600k @ 4.2GHz
Memory: 16GB DDR3 XMS3 1600MHz
Graphics Card: AMD Radeon HD 6870 1GB

Email/Gtalk:
[email protected]