A few more data points: MacBook Air, Core i5, dual core with hyperthreading.

Chrome 27: 1 or 2 cores. Chrome 26 gave me 3 a few times.

Firefox 21: 8-16 cores. One times out of four, the test goes up to testing 32 cores, but the CPU activity drops abruptly, and the test hangs (the button text remains a greyed out "Running").

Safari 6.04: 1 core. Rarely two.

The thing that surprised me most is that on my colleague's system (the second Arch x64 one), the estimator went up to 16 cores before dropping back to 14, while my own system (Mint 14) and the other Arch system both went to 8, then back to 6.

on my i7 (4 physical, 8 virtual cores, win xp 64bit, opera), it's now at 256 cores and testing for 512.. no end in sight ^^

I get 2-3 cores for a quad-core i7, Chrome on Windows 7. It should probably be 8.

T420, I7 with 2 Cores, HT and TB. Firefox on Ubuntu reports 6 cores.