Thanks for the reply.
It is failing after I have written everything, and am trying to boot from the internal NAND. I flashed using loadaddr 0x2000000, mtd1Size 0x300000, and filesize 0x44ad000. All those arguments I got straight from the documentation (USB Flash Recovery from U-Boot-Rev1.2.pdf). Are there different values I should try?
I just tried booting with the bootargs you suggest (which I believe are the same as in the documentation). Still nothing.
I went through the entire process again (except for re-upgrading u-boot) and saved a log of it, which you can download here:http://djpandemonium.com/sheevaflash.txt