It was only a matter of time.
Recently, it was reported that Windows RT had been jailbroken, allowing unsigned desktop apps to be run on the device. After an investigation, Microsoft issued a statement that applauded the efforts of the jailbreaker clrokr after determining that the jailbreak "is not a security vulnerability and does not pose a threat to Windows RT users." However, Microsoft also stated that there were no guarantees that the company wouldn't patch the exploit sometime in the future.
From then on, it was only a matter of time before the exploit reached the hands of the general public. XDA Developers member netham45 made public a batch file that simplifies the process of the jailbreak. clrokr's exploit involved modifying a value to alter the minimum signing level of Windows RT, allowing unsigned apps to run. Unfortunately, this value must be adjusted in the memory as the OS is running, as the value in the kernel cannot be adjusted, therefore the exploit must be repeated with every reboot.
This exploit can stand so long as Microsoft doesn't release an update to patch the issue. From the statement that Microsoft's released, it doesn't seem like an urgent issue for the company. After all, the exploit is limited because it's impermanent, and, as netham45 points out, useless since RT users can roll back to a previous version using recovery partitions.