As indicated earlier, Kragorn's procedure no longer worked after my NAND was erased. Working on a hunch, I tried the workaround before performing the image load and was finally successful getting uBoot to load. The log below shows the first time I tried the load_image that failed, followed by the halt/reset workaround and the 2nd successful attempt.
$this->bbcode_second_pass_code('', '> load_image uboot.bin 0x800000
524288 bytes written at address 0x00800000
downloaded 524288 bytes in 64.262405s (7.967 kb/s)
> resume 0x800200
> nand probe 0
NAND flash access requires halted target
NAND flash access requires halted target
NAND flash access requires halted target
NAND flash access requires halted target
NAND flash access requires halted target
NAND flash access requires halted target
unknown NAND flash device found, manufacturer id: 0x00 device id: 0x00
probing failed for NAND flash device
> halt
Halt timed out, wake up GDB.
target state: halted
target halted in ARM state due to debug-request, current mode: Supervisor
cpsr: 0x000000d3 pc: 0xffff0000
MMU: disabled, D-Cache: disabled, I-Cache: disabled
> sheevaplug_init
> load_image uboot.bin 0x800000
524288 bytes written at address 0x00800000
downloaded 524288 bytes in 65.744537s (7.788 kb/s)
> resume 0x800200')
PHEW...
$this->bbcode_second_pass_quote('', 'I') used the memory boot option and flashed mtd0 inside Pogoplug.
I reset the Dockstar and am booting from mtd0 now.