I did a fresh git clone of u-boot-xlnx (commit c55c35bb239a40d5d5d7678fc38b92c77cf882f8) and built u-boot.elf for the ZC-706 + FMCOMMS2. I made a Zynq boot image in SDK with u-boot.elf, a customized .bit file, and zynq_fsbl.elf and attempted to boot the system using a 8 GB SD card.
I observed the U-Boot error described at the bottom of this page:
fatload mmc 0 0x3000000 uImage from the u-boot command line also resulted in:
Controller never released inhibit bit(s).
** Bad device mmc 0 **
I remade the Zynq boot image using the u-boot.elf dated 3/22/2013 included with the ADI Zynq image
The system booted without incident.