Android L Developer Preview has been rooted
Less than 24 hours after the Android L developer preview images were released by Google, SuperSU developer Chainfire has managed to gain root access on it.
While Chainfire was previously worried that the next version of Android might effectively kill root, his findings have turned out to be wrong since the process pretty much remains the same with only a minor modification required to SuperSU.
According to Chainfire, gaining root access on Android L only requires moving the install recovery script from the /system/etc partition to /system/bin. Since Chainfire is is currently abroad, he has not been able to release an updated ZIP of SuperSU that will work on Android.
A good samaritan, however, has uploaded a modified SuperSU ZIP here, which you can flash from recovery to gain root access on your Nexus 5 or 7 running the developer preview of Android L.