Depending on how you have your machine configured, it goes all over the place. It doesn't make sense to me that the 2.3 would run substantially cooler than the 2.2 unless the 2.2 in question is defective. The 2.3 presumably has slightly better silicon but runs more cache memory. Surely it's a wash. In any case, for me on my 2.2GHz, I get 35C CPU battery-only idle, 37-39C using it (better performance if on a desk rather than my lap), 39C mains idle, in the mid 40s mains while charging, or even on battery if I've got a really large file on in Office, in the upper 40s mains with an external display and wired keyboard, in the mid 50s mains idle with an external display and flash use, in the 70s for doing time-series analysis on a big dataset with the single-processor version of Stata, and up to the mid 80s running Aperture's face detection feature on a big photoset. I simply don't think people understand how dramatically the temperature output on these machines varies with how you configure it, and with more and more people using laptops as desktops, these issues arise.
Then there's the whole question software and firmware glitches. Airport usage is very power-efficient and cool-running on these machines compared to past efforts, but with really inconsistent performance that some have speculated is due to aggressive power management. I've occasionally had odd behavior with the Quicktime plugin for Dashboard staying on when it shouldn't, and that has definitely made a few degrees' Celsius difference.
Lastly, how do you use the computer? One thing, NickZac, judging from the iStat screen shot you posted a while bag and considering how you've described the way you use the computer is you're running on battery without an external display and presumably without any peripherals attached. And then there's the whole question of how much memory you have, with the more you have meaning more power usage.
What I do know is that in every single category that I've compared it with my Early 2008 Core2Duo Penryn MBP, the 2011 2.2GHz quad-core runs faster at a lower temperature with lighter fan use.
I think it's important though that people do comparisons on these kinds of points because they can make a difference to how you use the computer. The computer manufacturers simply don't do very much about explaining differences between the configurations. If it turned out that, say, one of the 2.3 or 2.2 were cooler and quieter than the other, it would make a big difference for musicians who need to avoid background noise in a recording environment, or for those who depend on the battery, and in the long run also for anyone who places a premium on durability.
