If you want to install/run Wheezy from scratch I recommend to have a look at Martin Michlmayr's instructions starting with an u-boot update/upgrade http://www.cyrius.com/debian/kirkwood/sheevaplug/uboot-upgrade/
. After this continue with the Wheezy installation described at http://www.cyrius.com/debian/kirkwood/sheevaplug/install/
.BTW: Does sb have an actual DENX u-boot version which works for the standard SheevaPlug? I tried to compile my own version, but the standard DENX has no SD card support!? And I got stuck in trying to apply some Marvel patches to the DENX version.