Tried switching from Dalvik to ART from developer tools options and I started experiencing the same issue after rebooting into ART.
com.android.system.ui crash error, Lock Screen loop, could not get past the settings screen while trying to tap on the screen in the milli second window the phone responds after unlocking even after multitude times, and yes I had flipboard installing on my phone.
Fortunately for me, had G-Cloud backup and Air Droid installed, logged into both from my PC took a backup of all items on the phone and did a Factory Reset to bring it out of the ventilator
So for those of you who are asking for ways to backup, you get lucky if you have airdroid installed on your phone prior to this episode, if you count yourselves as the lucky ones then simply login to airdroid website and access your phone through your Wi-Fi (make sure both are connected in LAN mode to the same network) and download your data from there on.
Also don't try removing flipboard from an external source, your phone will simply do a reboot and you are back to square one again.
Since there is news of Dalvik being dumped for ART, its high time they come up with a solution for the future of the phone.