In recent months, sources reported on several advancements and design changes expected with the first Intel iBooks, but were for the most part mum on specifically which Intel processors the notebooks would include. (Quite possibly because this spec may be loose and subject to change at the drop of a dime, as today's announcements have proven.)
Based on marketing logic and the fact that Apple's professional and consumer notebooks have long been distinguished by a variation in their processor specs, AppleInsider had derived that the iBooks would utilize single-core Intel Core Solo processors, as outlined in a recent report.
The 1.67GHz Intel Core Duo processor is currently the slowest dual-core chip on Intel's new mobile processor roadmap, meaning that iBooks would have likely had to sport a single core 1.67GHz Core Solo processor to differentiate themselves from a 1.67GHz Core Duo MacBook Pro. Now that Apple has shelved plans for a 1.67GHz Core Duo MacBook Pro, instead bumping this model to 1.83GHz free-of-charge, this may not be the case.
According to Intel's price listing for the Core Solo and Core Duo, Apple stands to save only $32 dollars by opting to include a 1.67GHz Core Solo processor, rather than a 1.67GHz Core Duo, in any of its forthcoming products. The chips reportedly cost $209 and $241, respectively. On the other hand, Intel's 2.16GHz Core Duo processor costs approximately $217 more than the 2.0GHz model, somewhat justifying Apple's $300 upgrade fee from 2.0GHz to 2.16GHz on the MacBook Pro.
Applying the same logic, it's also possible that Apple could pack the 1.67GHz Core Duo into its forthcoming Intel Mac mini desktop systems. However, since the Mac mini is entry-level system, this may be less likely.