Thanks Shootist007. It seems to be a bit more complicated than this.
I have just received a reply from Marcel Breisik, the developper of Hardware Monitor (that's what I call great support, and I had not even bought the software yet!) and here are his answers:
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> I have installed the latest demo version of Hardware Monitor (4.96) and it seems to work fine. However, it doesn't seem to be able to display the CPU frequency change in real-time due to Speedstep or Turboboost action. The clock frequency is stuck at 2.9GHz, the nominal frequency.
This is the intended behavior for Hardware Monitor when running on Intel Core i processors.
A live update of the CPU clock frequency is only supported via artificial software sensors provided by Hardware Monitor for certain PowerPC systems, and the first generation of Intel Core Solo and Core Duo processors.
> I have checked in Windows/Bootcamp with CPU-Z or Intel Turboboost Monitor, and in Win7 the frequency changes as expected between 1GHz when idle to 3.6GHz when one core is under max load and the others are on idle.
The System Information window is not designed to be updated in real-time. Unfortunately, it is generally impossible to determine actual "live" frequency values for TurboBoost when running Mac OS X.
The Mac OS X kernel intentionally blocks the use of certain processor features which are needed to compute the current frequency. You would need to replace the OS kernel or run other operating systems to get such values.
> I am of course assuming that Speedstep and Turboboost are enabled in 10.7.4 (Lion), but I have no way to confirm this.
Hardware Monitor can basically confirm this if you have enabled its sensor driver for extended x86 monitoring. After that, the TurboBoost frequency support table will become visible when pressing the "More info" button in the System Overview window.
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So it looks like Turboboost is enabled but the CPU frequency can't be displayed in real-time on recent Macbooks.
I have asked for a confirmation that as long as you can display the Turboboost info in Hardware Monitor, it means that the feature is actually enabled in the OS (and HM doesn't just display generic info derived from the CPU type/model), but assuming it is the former it looks like it's just OS X which is preventing from reading the info, not the feature which is disabled.
Anyway, great software and great support, so I'm buying it even if it can't display the CPU speed in real-time...