Technically speaking? No, of course not; Win8 runs on x86/x64 (processors based on Intel's 80386 instruction set architecture, with or without AMD's 64-bit extensions). RT runs on ARM, a completely different processor instruction set. ARM processors, such as the Tegra 3 chip in Surface RT, can't run x86 code directly, so there's no way to run an x86 OS on them.
Now, with that said, there's really only one meaningful difference in the behavior of Win8 from that of Win RT: the requirement that desktop apps and libraries be signed by Microsoft. Without athat, you can compile programs for ARM, or run .NET code (which is processor-independent), with no problems at all. There *is* a way to enable this on RT: go look up the "jailbreak tool" on the dev&hacking sub-forum.
Note, however, that the amount of software available for RT thus far is limited. Pure .NET 4.x apps will usually work fine, but RT doesn't include older versions of the .NET runtime. Open-source apps that are compiled using Visual Studio can be pretty easily re-compiled for ARM, but ones that are compiled using GCC only are much more difficult, and closed-source apps can't be recompiled at all. We have limited support for both Java and Python, as well. There is a tool being developed that allows running x86 code on RT using dynamic recompilation to ARM code, but that tool is still in early beta phase and doesn't yet support many programs.