The XDA jailbreak won't let the Surface RT run x86 apps - the Surface RT simply doesn't have the hardware for it. What the jailbreak will do is let the Surface RT run Win32 apps that have been recompiled for ARM. Unless you have just such an app already available, or have the source code and the appropriate cross-compiler and the knowledge to use it, the jailbreak won't do much for you.
Windows on a non-x86 CPU isn't really novel or special. When it first came out, Windows NT was available for Intel 80386, DEC Alpha, and MIPS R3000, and was later ported to the Itanium CPU where it has been available for years now. ARM is simply the most recent CPU to get a port of Windows. Windows NT was actually written on the MIPS R3000, then ported over to x86 and Alpha - Microsoft did this deliberately to ensure that Windows NT was cross-platform from the beginning. I've personally run Windows on DEC Alpha and MIPS R3000, in addition to the more usual x86 machines and now ARM. But for each of these different Windows systems, you could only run applications written for that specific CPU architecture, you couldn't just run any old x86 app on it.
What Microsoft has done with the Surface is basically the same as with the Alpha, MIPS, and Itanium versions of Windows - they cross-compiled the related Win32 apps for the specific CPU (Alpha, MIPS, Itanium, ARM). The difference is that with those older versions of windows you could still install your own Win32 apps.
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