Instead, they may simply be dropping support for 32 bit apps on a 64 bit CPU. Having to support 64 bit and 32 bit apps one a single device forces them to ship two versions of every shared library, and is probably annoying for them in various types of interprocess communication, because, for example, CGFloat and integer types are different sizes.
My guess is that they will release a 32 bit version of iOS and a 64 bit version, and each one will only run apps for the native processor word size.