Or scroll around for locked topics. Or check the FAQ at the top of the page. Basically, if it proves possible to port NS in a timely manner and such, I think the devs would be delighted to use Source. But they did a lot of backflips to make NS work on the HL1 engine and Source isn't guaranteed to be any more forgiving.
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<span style='color:red'>***Locked.***</span>