As of my experience from my last two Vegas visits (within the last two months), it is open to just about everyone... They actively use the 'first class' lane to balance lines so effectively, there are two lines which are both the same...

I've never seen any noticeable lines at T3 so all is relative - but then, so is the need to clog up the 'first class' lane with non-elite passengers...

Just an irritating way of managing it - basically making the 'first class' lane obsolete.