So I just downloaded the IPSW from the dev portal. Synced and backed up my iPhone 5S on the most recent beta build of IOS 8. Then the magic moment to manually upgrade caused iTunes 12 (latest on Yosemite) to crash. Now my phone is stuck in DFU mode, and iTunes will crash immediately if my phone is plugged in at all. So be warned. I'm switching to my mavericks partition to try to install off the latest iTunes there. Update to follow.