If you look at the RISC OS Six Home Page, it says that "it is now based on a completely 26 bit / 32 bit neutral source". This is does not necessarily mean that all future builds will be 32 bit.
The preview is advertised as being for "RiscPC, A7000 and VirtualRPC" users. None of these platforms would benefit from a 32 bit build of the OS (it would just prevent you from running old applications). Therefore I assume that the preview is a 26 bit build.