I've read build instruction from mavell documents set. Basically philipl's instruction is correct.
1) tar jxvf u-boot-1.1.4.tar.bz2
2) mv u-boot-1.1.4 u-boot-3.4.16
3) unzip u-boot-3.4.16 (this will overwriting existing files)
4) cd u-boot-3.4.16
5) tar zxvf sd.tgz
6) Copy attatched mv_kw.h to include/configs/mv_hw.h
7) make rd88f6281Sheevaplug_config NBOOT=1

make clean; make
And then you will get u-boot-rd88f6281Sheevaplug_400db_nand.bin
9) copy u-boot-rd88f6281Sheevaplug_400db_nand.bin to /tftpboot/u-boot.bin
(Assume that you setup tftp server)
In the sheevaplug u-boot prompt, 'bubt' command will update u-boot.bin image on the nand flash.
10) reset sheeva plug
And enjoy youself. I also attached prebuilt u-boot.bin image.