Well, I don't think Apple will use a single-core chip for the next iBooks. I am adamant. Why?
1- ThinkSecret reported, and sticks to its report, that iBooks are DEFINITELY coming in January, most likely to be announced this Monday. The single-core Yonah chip won't ship until March, earliest, according to sites that have published the Intel roadmap... Will Apple use the current, outdated Centrino (I know Yonah is also the Centrino I don't know the name of the current chip)? I highly doubt it. The current Centrino does not support the trusted-computing technology (it has a name, I forget what it is), and that doesn't fit with Apple's gameplan. Plus, Intel must be phasing out production of this line...
People cite cost as a factor. As MacinDoc accurately points out, it's a $32 difference between the single-core Yonah and the dual-core Yonah. This is a much smaller price gap than we saw with the Celeron M and the Pentium M when they debuted (compare $130 with the base Celeron chip to something like $250 for the M). If Apple is going to be using Yonah at all, it will be the dual-core variety. It's simply more cost effective- they can cut $30 out of their costs anyway just by using off-the-shelf chipsets, etc, now that they are moving to PC parts. Plus, there is a lot of talk that they negotiated a better price than other PC manufacturers from Intel because Apple can showcase Intel technology in a better way than Dell...
Something that I do find interesting is how the rumor sites, this included, haven't even speculated as to the exact specs of the iBooks or Minis. Usually, by this time (four days before the keynote), ThinkSecret and MacRumors would have some sort of analysis and price points. Odd.
-Jim