According to id Software, this threaded renderer will shine on systems with more than two cores, so really just quad-core and octal-core systems (for now).
You misunderstood what the guy from Id was saying. The threaded renderer does give a performance increase on dual core systems but it was expected to give a greater increase on 4+ cores. This is shown in the results of numerous benchmarks of the window$ 1.2 client and in my own benchmarks of the 1.4 Linux client. If you have a dual core system and you aren't seeing a noticeable improvement then you weren't CPU limited to begin with. That's why people with high end c2d and c2q systems aren't seeing much of an increase.
So far I've only tested at the settings I play at. I'll do some low res totally CPU limited tests but for now here are my results:
Opteron 165 @ 2727mhz
2gb DDR550 3-4-4-8 1t
8800GT core 700mhz memory 2000mhz 512mb
Ubuntu 7.10 x86-64 2.6.22-14-rt kernel
Nvidia 169.07 drivers
"checkpoint 1.4" demo 1600x1200 all in-game settings set to maximum 16xaf 2xaa
threaded renderer '2'
51.3 fps
threaded renderer '0'
42.7 fps
It might not seem like much of an increase but it improved FPS where it really counted. With the threaded renderer enabled I never see under 30fps and it drops to 30 less often.