I believe I've found a fix for the USB boot failures when multiple devices are attached to the Sheevaplug via a hub. Looks like U-Boot was incorrectly determining the speed of the devices as it was probing the bus. Attached is a modified usb.c that can be applied to your build tree as well as a prebuilt flash image.
Please let me know if you have issues with this change. You know how newbie posters can be
FYI, I used the steps provided by http://plugcomputer.org/plugforum/index.php?topic=765.0
for setting up a U-Boot build.