Microsoft's newest Windows comes in two flavors: one normal, full version, Windows 8, and one for ARM processors, Windows RT. The latter only allows consumers to run apps from the Windows Store or desktop apps signed by Microsoft.
Clrokr, let's call him/her a hacker, has been able to disable this security check to install every application that would also run on a fully fledged Windows 8. Unfortunately, this hack will only work as long as you don't reboot. After each reboot, the hack has to be applied again. Details about the hack can be found in a special blog.
If you're really interesting in applying this hack, then there is an easier way than following the process from the mentioned blog. A developer over at XDA developers created a tool: RT Jailbreak Tool that you can run each time after you've started your Windows RT device.
I will not apply this Jailbreak because even though I'm disappointed my Windows RT tablet is only able to run Windows Store apps (besides Office and some dev tools ;), I'm not sure if it's worth the trouble.
It would be nice though to run real applications on the desktop because, beside Office which is not that user friendly in touch mode (it's possible but…), the desktop is pretty useless on Windows RT tablets. Even though Microsoft congratulated the hacker, they also announced to fix the problem that allows this security breach. So it looks like Windows RT is going to stay "dumb" – Store apps are nice but I already found some things that you can't do with the WinRT API (or at least is not as simple or powerful as on other platforms). More on those in other blog posts.
If someone actually tried the jailbreak, I would like to read more about it – just comment or contact me directly :)