Upgrade to mainline U-Boot from Debian archive
When Marvell originally released the first plug computer, they created their own version of u-boot with support for their new devices. Unfortunately, this version of u-boot is fairly out of date nowadays compared to mainline u-boot and has several problems. Support for plug computers (such as SheevaPlug and GuruPlug) have been integrated into the mainline u-boot (also known as DENX u-boot) in the meantime and Clint Adams has packaged it for Debian.
I finally found the time to test Clint's u-boot binary on my devices and have updated the SheevaPlug installation guide accordingly. If you're have installed Debian to a SheevaPlug according to my instructions, I suggest you upgrade.
If you boot from a MMC/SD card, you should be aware that the mmcinit command has been renamed to mmc init in order to be consistent with the naming of other commands. You'll therefore have to update your bootcmd_mmc variable in u-boot like this:
setenv bootcmd_mmc 'mmc init; ext2load mmc 0:1 0x00800000 /uImage; ext2load mmc 0:1 0x01100000 /uInitrd' saveenv