I think what earlier posters were suggesting is that the base frequency of the 5Y31 chip that Apple has set to 1.1GHz has been achieved by Apple using a higher voltage than is being used for the 5Y51 (1.2GHz) and 5Y71 (1.3GHz) chips. Those chips achieve their base frequencies with a lower voltage (4.5) than the low-end chip (6), so the low end chip, while marginally slower at the bottom end, may actually run hotter during regular use. Of course, the TurboBoost mode, wherein the CPUs ramp up their processing frequencies, happens by increasing the voltage, and thereby the heat produced by the chips (all of them). Gaming or graphically intensive applications require higher frequency, requiring higher voltage, producing more heat.
So, if you don't need the high-end power (for games or graphics), the 1.1 is probably plenty powerful, since it turbos up to 2.4GHz. But there's a chance (theoretically) that in sustained ordinary, everyday use closer to the base frequency, it may run warmer than the 1.2 and 1.3 GHz chips, which use less voltage for their base frequencies.
Frankly, this is very theoretical and possibly not even noticeably true. We won't know until someone gets their hands on the various models and tests it. My advice is to simply determine your usage needs and order appropriately. If you play games or do lots of video editing, etc... you might look toward the 1.3GHz model. Then again, if that's your usage, you're probably better off with a different model (MacBook Pro) altogether. If you don't do gaming and graphics, the 1.1 or 1.2 models are likely plenty adequate for you. Then the issue is money, and the extra cost of the 1.2 model will likely be more justified by the larger SSD it comes with.